THE
ELEMENTS OF. EMBRYOLOGY.
BY
M. FOSTER, M.A., M.D., F.R.S.,
FELLOW OF AND PRJELECTOR IN PHYSIOLOGY IN TRINITY COLLEGE, CAMBRIDGE,
AND
FRANCIS M. BALFOUR, B.A.,
FEUOW OF TRINITY COLLEGE, CAMBRIDGE.
UonUon :
MACMILLAN AND CO.
1874.
[All Eights reserved.]
F4
OGY
LlfcBART
a
Camfcrftge :
PRINTED BY C. J. CLAY, M.A.
AT THE UNIVERSITY PRESS.
TO
THOMAS HENEY HUXLEY
AS A LITTLE TOKEN
OF OUR APPRECIATION OF HIS WORTH
AND OF
HIS MUCH KINDNESS TO OURSELVES
THIS BOOK
IS RESPECTFULLY DEDICATED BY
' E ATJTHOKS.
PKEFACE.
IN this volume we offer to the public the first part of what
we hope may serve as a systematic introduction to the study
of Embryology. Some apology is perhaps necessary for the
separate publication of a part only of the whole subject; but
we trust that the following reasons will justify the course we
have adopted.
Those who have paid attention to recent embryological
researches must be aware of what we may venture to call
the tumultuous condition of many parts of the subject, and
of the extreme difficulty in many cases of forming a clear
and decided judgment without the aid of independent
observations. It is this necessity of having repeatedly to
work over contested points with a view to reconcile
diametrically opposed .statements, or to verify startling
announcements, which has rendered so laborious the task
we have undertaken, and which so much delays its com-
pletion.
On the other hand, whoever wishes to have a sound
foundation of embryological knowledge cannot do better
than gain a thorough insight into the development of the
bird. The practical advantages offered by the hen's egg
62
viii PREFACE.
altogether outweigh the theoretical objections to beginning
with the avian type. In many respects, it might be thought
desirable to commence with a holoblastic ovum ; but the
large food-yolk of the bird's egg is in many ways a great
assistance to the study of changes going on in the blasto-
derm. The chick is of all embryos the best to begin with ;
when its history has once been mastered, the subsequent
study of other forms becomes an easy matter.
We venture to hope therefore that we shall meet with
general approval, in having described at considerable length
the history of the chick, and in hastening the publication of
our account, by bringing it forward in a separate form.
In the earlier chapters, especially, we have gone into
very considerable detail; and in order to make the account
intelligible to the beginner, have not been deterred by the
fear of wearying our readers with elementary and recapitu-
latory statements. Debated matters and details of minor
importance have been put in small print ; these may be
omitted by the student in reading the book for the first
time. Though we have sometimes introduced names in
connection with important observations, we have not thought
it necessary to do this systematically. For recent or debated
statements however, the authorities .are always cited.
The worth of such a book as this will be very small if
the student simply contents himself with reading what is
written; and to facilitate the only really useful mode of
study, that of actual observation, a few practical instructions
have been added in an appendix.
The readiness with which the development of the skull
can be studied in the chick renders it, in spite of obvious
PREFACE. ix
objections, a suitable introduction to the important subject
of cranial morphology. It is with this view that we have
given a separate chapter on the skull, which we hope may
serve as an introduction to the study of Mr Parker's elaborate
memoirs.
In the remaining parts, which we shall do our best to
complete as soon as possible, the several histories will be
treated with much greater brevity, and much more space will
be given to theoretical considerations.
The figures, whose source is not acknowledged in the
text, were drawn by Miss A. B. Balfour, except a few by
ourselves.
The drawing on wood was executed partly by Mr Allchin,
but chiefly by Mr Collings ; and all the drawings were cut
by Mr Cooper. We have to thank those gentlemen for the
trouble they have taken in a matter in which, for many
reasons, the result never seems commensurate with the
labour. We are much indebted to Professor Huxley for
having kindly looked over the proofs of the Chapter on the
Skull.
The work took its origin in a course of lectures delivered
by myself, but many causes prevented my taking the task
seriously in hand, until I was joined by my friend and
former pupil Mr F. M. Balfour, whose share in the matter
has, to say the least, been no less than my own.
M. FOSTEK.
TABLE OF CONTENTS.
INTRODUCTION ... , pp. i 10.
PART I. THE HISTORY OF THE CHICK.
CHAPTER I.
THE STRUCTURE OP THE HEN'S EGG, AND THE CHANGES WHICH TAKE PLACE
UP TO THE BEGINNING OP INCUBATION . . pp. II 26.
i. The shell, i. The shell-membrane. 3. The albumen. 4. The vitelline
membrane. 5. The yolk. 6. The yellow yolk. 7. The white yolk. 8. The
white yolk-spheres. 9. The structure of the blastoderm. 10. Recapitulation,
n. The ovarian ovum. 12. The descent of the ovum along the oviduct.
13. The impregnation of the ovum. 14. Segmentation. 15. The formation of
the upper and lower layers.
CHAPTER II.
A BRIEF SUMMARY OP THE WHOLE HISTORY OP INCUBATION, pp. 2742.
i. The embryo is formed in the area pellucida. -2. The epiblast, mesoblast,
and hypoblast. 3. The extension of the blastoderm over the yolk. 4. The
vascular area. 5. The head-fold and the other folds by means of which the
embryonic sac is formed. 6. The outward shape of the embryo. 7. The
formation of the neural tube and alimentary canal: somatopleure and splanch-
iiopleure. 8. The amnion. 9. The allantois.
Xll CONTENTS.
CHAPTER III.
THE CHANGES WHICH TAKE PLACE DURING THE FIRST DAY OF INCUBATION,
PP- 4357-
I. Variations in the progress of development. 2. The embryonic shield.
3. The formation of the epiblast, mesoblast and hypoblast. 4. The primitive
streak, the primitive groove. 5. The head-fold, the medullary groove, me-
dullary folds, and notochord. 6. The amnion ; the changes taking place in the
three layers. 7. The increase of the head-fold. 8. The closure of the me-
dullary canal. 9, 10. The cleavage of the mesoblast : formation of splanchno-
pleure and somatopleure. ir. The protovertebrse. 12. The formation of the
vascular area. 13. Recapitulation.
CHAPTER IV.
THE CHANGES WHICH TAKE PLACE DURING THE SECOND DAY, pp. 58 83.
I. Increasing distinctness and prominence of the embryo. 2. The first cere-
bral vesicle. 3. The increase in the number of proto vertebrae. 4. The first
rudiments of the alimentary canal. 5. The formation of the heart. 6. The
formation of blood-vessels ; the omphalo-mesaraic veins and arteries, the sinus
terminalis. 7. Changes taking place in the cells of the several layers.
8. The rudiment of the Wolffian duct. 9. Recapitulation of the changes
during the first half of the second day. 10. Increasing prominence of the
embryo ; the tail-fold and the lateral folds. 1 1. Continued closure of the
medullary canal. 12. The optic vesicles. 13. The second and third cerebral
vesicles. 14. Change of position of the optic vesicles. 15. The vesicles of the
cerebral hemispheres. 16. The cranial flexure. 17. The rudiment of the ear, or
auditory sac. 18. Changes in the heart. 19. The primitive aortse and first
pair of aortic arches, the omphalo-mesaraic vessels, the sinus terminalis.
20. The second and third pair of aortic arches. 11. The Wolffian duct.
22. The amnion. 23. Recapitulation.
CHAPTER V.
THE CHANGES WHICH TAKE PLACE DURING THE THIRD DAY, pp. 84 140.
I. The diminution of the albumen. 2. The spreading of the opaque and
vascular areas. 3. The vascular area. 4. The continued folding in of the
embryo. 5. The increase of the amnion. 6. The change in the position of
the embryo. 7. The curvature of the body. 8. The cranial flexure.
9* Growth of the vesicles of the cerebral hemispheres; the third ventricle,
pineal gland, infundibulum and pituitary body, the cerebellum and medulla
oblongata. 10. Changes in the spinal cord. n. The formation of the eye.
Histological changes in the retina, optic nerve, and lens. 12. The formation
of the ear. 13. The nasal pits* 14. The visceral clefts and folds. 15. The
aortic arches. 16. Changes in the heart; the Ductus Cuvieri and cardinal
veins. 17. The folding in of the alimentary canal; the formation of the tail.
1 8. The lungs. 19. The liver. 20. The pancreas and spleen. 21. The
thyroid body. 22. Changes in the trunk of the embryo. 23. Separation of
the muscle-plates from the protovertebrse. 24. Growth of the intermediate
cell-mass. 25. The cranial nerves. 26. The Wolffian duct. 27. Recapi-
tulation.
CONTENTS. xiil
CHAPTER VI.
THE CHANGES WHICH TAKE PLACE DURING THE FOURTH DAY, pp. 141 173.
j. Appearances on opening the egg. 2. Growth of the amnion.
3. Narrowing of the splanchnic stalk. 4. Increase in the cranial flexure.
5. The first appearance of the limbs. 6. Growth of the head. 7. Changes
in the nasal pits. 8. Formation of the mouth. Q. The cranial nerves.
10. .The allantois. n. Changes in the protovertebras ; the spinal ganglia.
12. The secondary segmentation of the vertebral column. 13. Changes in
the notochord. 14. Ossification of the vertebrae. 15. The ribs. if>.
Changes in the muscle-plates. 17. The Wolffian body and duct. 18. The
duct of Mu'ller. 19. The kidneys. 20. The ovaries and testes. i\.
Changes in the arterial system. 22. Changes in the venous system; the
veins of the liver. 23. Changes in the heart; the ventricular septum.
24. Recapitulation.
CHAPTER VII.
THE CHANGES WHICH TAKE PLACE ON THE FIFTH DAY, pp. 174 199.
i. Appearances on opening the egg. i. The growth of the limbs.
3. The cranium; the investing mass and trabeculae. 4. Changes in the
face ; formation of the nose and nasal passages. 5. Appearance of the anus.
6. Changes in the spinal cord ; the formation of the grey and white columns,
and of the posterior and anterior fissures. 7. Changes in the heart; the
rudiment of the auricular septum, the division of the bulbus arteriosus into
aorta and pulmonary artery, the formation of the semilunar valves. 8. Changes
in the heart during the sixth day. 9. Subsequent changes in the heart; the
completion of the auricular septum, the arrangement of the openings of the
venae cavte. 10. Histological differentiation; the fate of the three primary
layers, u. Recapitulation.
CHAPTER VIII.
FKOM THE SIXTH DAY TO THE END OF INCUBATION, pp. 200 224.
r. The commencement of distinct avian differentiation. 2. The foetal
appendages during the sixth and seventh days. 3. During the eighth, ninth
and tenth days. 4. From the eleventh to the sixteenth day. 5. From the
sixteenth day onwards. 6. The changes in the general form of the embryo
during the sixth and seventh days. 7. During the eighth, ninth and tenth
days. 8. From the eleventh day onwards ; feathers, ossifications. 9. Changes
in the venous system before and after the commencement of pulmonary respi-
ration. 10. Changes in the arterial system, the modifications of the aortic
arches, u. Summary of the chief phases of the circulation. 12. Ex-
clusion from the egg.
xiv CONTENTS.
CHAPTER IX.
THE DEVELOPMENT OF THE SKULL, pp. 225 238.
i, 2. The primordial cranium. 3, 4. The investing mass of Rathke.
5. The trabeculse cranii. 6. The cartilages of the first visceral arch. 7.
The maxillary process. 8. The mandibular arch. 9. The hyoid arch. 10.
The cartilages of the third visceral arch. 1 1. Changes in the cranium during
the fifth and sixth days. 12. During and after the seventh day. 13. The
condition of the cranium at about the middle of the second week. 14. Ecto-
steal and endosteal ossifications of the cartilaginous cranium. 15. Formation
of the membrane bones. 16. Progress of ossification during the second and
third weeks. 17. Fenestration of the ethmo-presphenoid cartilage. 18. Ossifi-
cations in the prootics and alisphenoid. 19. Changes in the basitemporals.
Formation of the vomer. 20. The changes which take place immediately
after exclusion from the egg. 21. Further changes in the splint bones.
Coalescence of the bones after birth. Table of bones classified according to
their mode of ossification.
APPENDIX.
PEACTICAL INSTRUCTIONS FOE STUDYING THE DEVELOPMENT OP THE CHICK,
pp. 239267.
I. Incubators. II. Examination of a 36 to 48 hours embryo. III.
Examination of an embryo of about 48 50 hours. IV. Of an embryo at the
end of the third day. V. Of an embryo of the fourth day. VI. Of a
blastoderm of 20 hours. VII. Of an unincubated blastoderm. VIII. Of the
process of segmentation. IX. Of the later changes of the embryo. X. Study
of the development of the blood-vessels.
ERRATUM.
p. 124, in the description of Fig. 39 B, for 'Superior vertebral' substitute
'Jugular.'
LIST OF ILLUSTKATIONS.
FIG. PAGE
1. DIAGRAMMATIC Section of an Un incubated Fowl's Egg . . .12
2. A. Yellow yolk-sphere filled with fine granules. J5. White yolk-
spheres and ^spherules of various sizes and presenting different ap-
pearances '... " ' , * V , . . . . . 15
3. Section of a Blastoderm of a Fowl's Egg at the commencement of
Incubation * * . . . . . . 18
4. Section througli the Germinal Disc of the ripe Ovarian Ovum of a
Fowl while yet enclosed in its Capsule . . . . . 19
5. Surface Views of the early Stages of the Segmentation in a Fowl's
Egg . . , . . .22
6. Surface View of the Germinal Disc of a Hen's Egg during the later
Stages of Segmentation . . . ; . . . 23
7. Section of the Germinal Disc of a Fowl during the later Stages of
Segmentation . . . . . , '. : . . . 24
8. A to N. A series of purely diagrammatic representations introduced
to facilitate the comprehension of the manner in which the body of
the embryo is formed, and of the various relations of the yolk-sac,
amnion and allantois ........ 29 32
9. Diagrammatic Longitudinal Section through the Axis of an Embryo. 33
10. Section of a Blastoderm at right angles to the long axis of the
Embryo after eight hours' Incubation ...... 46
XV i LIST OF ILLUSTRATIONS.
FIG.
11. Surface View of the Pellucid Area of a Blastoderm of 18 hours . 49
12. Transverse Section of a Blastoderm incubated for 1 8 hours . . 51
13. Transverse Section through the Dorsal Region of an Embryo of the
Second Day , . : , . V . * ... V, . 55
14. An Embryo Chick of the First Day (about thirty-six hours) viewed
from below as a transparent object . '. '. ' A , . ... 59
15. Embryo of the Chick at 36 hours viewed from above as an opaque
object . . ..''.. 6 1
1 6. Diagrammatic Longitudinal Section through the Axis of an Embryo. 62
17. A, B. Two consecutive Sections of a 36 hours Embryo illustrating
the formation of the heart . . . . . . . .67
1 8. Transverse Section of an Embryo at the end of the Second Day
passing through the region of bulbus arteriosus .... 68
19. Surface View from below of a small portion of the posterior end of
the pellucid area of a 36 hours Chick * . . . . 70
20. Transverse Section through the Dorsal Region of an Embryo of 45
hours ............ 74
21. Embryo of the Chick at 36 hours viewed from above as an Opaque
Object , . ' ' 75
22. Head of a Chick at the End of the Second Day viewed from below
as a Transparent Object . . . . . . . .76
23. Diagram of the Circulation of the Yolk-Sac at the end^of the Third
Day of Incubation . . . . . ... .85
24. Chick of the Third Day (54 hours) viewed from underneath as a
Transparent Object i . . . 88
25. Head of a Chick of the Third Day viewed sideways as a Transparent
Object . ' s . . . . .... . 90
26. Section through the Hind-Brain of a Chick at the end of the Third
Day of Incubation . . . . . . * : 'i .-... . * jr 95
27. Diagrammatic Sections illustrating the Formation of the Eye . . 97
28. Diagrammatic Section of the Eye and the Optic Nerve at an early
.stage .... ' ....,', *';*''.;. ,' .'. ' ,f V ,t . 4* 98
LIST OF ILLUSTRATIONS. XV11
FIG. PAGE
29. Diagrammatic representation of the Eye of the Chick of about the
Third Day as <seen when the head is viewed from underneath as a
transparent object .... f .... 99
30. D, E, F. Diagrammatic Sections of the Eye of the Chick of about
the Third Day . , " , . ; , . . . ; . . . . 100
31. Section of the Eye of Chick at the Fourth Day . ,-.' . * ':. . 104
32. Section through the Hind-Brain of a Chick at the end of the Third
Day of Incubation . . . . , . . . . no
33* Two Views of the membranous Labyrinth of Columba domestica.
A from the exterior, B from the interior ,,. * . . 112
34. Transverse Section of the Head of a Foetal Sheep (16 mm. in
length) in the region of the Hind-Brain . . t , 113
35. Section of the Head of a Foetal Sheep (20 mm. in length) . .114
36. Section through "the internal Ear of an Embryonic Sheep (28 mm.
in length) . ,. 'f, .,. .' . . . 116
37. Head of an Embryo Chick of the Third Day viewed sideways as an
Opaque Object . . . . . , . * ; - >; /., nS
38. The same Head as shewn in Fig. 37, seen from the Front . 121
39 A. Diagram of the Arterial Circulation on the Third Day . . 122
39 B. Diagram of the Venous Circulation on the Third Day '', ; . 124
40. Section of the Tail-end of an Embryo (Chick) of the Third Day . 125
41. Section through the Dorsal Ilegion of an Embryo at the commence-
ment of the Third Day ' ..*'* * "V, "*- * 126
42. Diagram of a portion of the Digestive Tract of a Chick upon the
Fourth Day. . ' . ..".".* ., , . . i^S
43. Four diagrams illustrating the Formation of the Lungs . . . 129
44. Section through the Dorsal Region of an Embryo at the end of
the Third Day . v . . ' , ~, >, . ."-\ . . 135
45. Head of an Embryo Chick of the Third Day (seventy-five hours)
viewed sideways as a transparent object : ' . . 137
46. Embryo at the end of the Fourth Day seen as a transparent
.object. . . . . . . . . . . .142
XV111 LIST OF ILLUSTRATIONS.
FIG. PAGE
47. Section through the Lumbar Region of an Embryo at the End of
the Fourth Day 144
48. A. Head of an Embryo Chick of the Fourth Day viewed from
below as an opaque object. B. The same seen sideways . .146
49. Longitudinal Section of the Tail-end of an Embryo Chick at the
commencement of the Third Day . . . . . .148
50. Longitudinal Section of the Tail-end of an Embryo Chick at the
middle of the Third Day .* - . . . . . . . 149
51. Section of the intermediate Cell-mass on the Fourth Day . .165
52. State of Arterial Circulation on the Fifth or Sixth Day .">'' 169
53. Diagram of the Venous Circulation at the Commencement of the
Fifth Day .''.,, . * , > . . ' * . 170
54. Heart of a Chick on the Fourth Day of Incubation viewed from
the Ventral Surface . * > ( * . . N * . 172
55. View from above of the Investing Mass and of the Trabeculae on
the Fourth Day of Incubation > *-. * . , . 177
e,6. A. Head of an Embryo Chick of the Fourth Day viewed from
below as an opaque object. B. The same seen sideways. . 180
57. Head of a Chick at the Sixth Day from below '* . . . . 181
58. Head of a Chick of the Seventh Day from below ... ... . 182
59. Section through the Spinal Cord of a Seven Days Chick . . 188
60. Two views of the Heart of a Chick upon the Fifth Day of Incuba-
tion . . .4 . V ( > " . ' .192
61. Heart of a Chick upon the Sixth Day of Incubation, from the
Ventral Surface . ,*'*-' . i \ . . 193
62. Diagram of the Venous Circulation at the Commencement of the
Fifth Day . . . < . '.. . * . . 206
63. Diagram of the Venous Circulation during the later days of Incu-
bation . . m * *" V % . . aoS
64. Diagram of the Venous Circulation of the Chick after the com-
mencement of Respiration by means of the Lungs . . .n
65. State of Arterial Circulation on the Fifth or Sixth Day . . . ii2
LIST OF ILLUSTRATIONS. xix
FIG. PAGE
66. Diagram of the Condition of the Arches of the Aorta towards the
Close of Incubation . 216
67. Diagram of the Arterial System of the Adult Fowl . . .219
68. View from above of the Investing Mass and of the Trabeculse on
the Fourth Day of Incubation . . . . . . .226
69. View from below of the Paired Appendages of the Skull of a Fowl
on the Fourth Day of Incubation .,*.',.. . . 229
70. Side view of the Cartilaginous Cranium of a Fowl on the Seventh
Day of Incubation * . * ,. . . . . . 231
71. Embryonic Skull of a Fowl during the Second Week of Incubation
(third stage) from below . . . . . . . .234
INTRODUCTION.
EVERY living being passes in the course of its life through a
series of changes of shape and structure. These changes
may, in their completest form, be considered as constituting
a morphological cycle, beginning with the ovum and ending
with the ovum again.
Among many living beings and especially among verte-
brate animals by far by the greater part of the life of the
individual is spent in one particular phase, which is not only
of longer duration than the rest, but also of much more
importance, inasmuch as during it the greater part of the
' work ' of the living being is done. This is generally spoken
of as the adult stage, and in most cases immediately precedes,
or is peculiarly associated with, the completion of the morpho-
logical cycle in the appearance of a new ovum.
The word embryology may be generally taken to mean
the study of the successive morphological phases through
which a living being passes from the ovum to the adult
stage, or the study of the gradual 'development' of the ovum
to the adult form ; though, especially among some of the so-
called lower forms of life, its meaning must be so extended
as to embrace all the morphological phases of an individual
life. Embryology is thus a part of and a necessary intro-
duction to the wider study of ' Generation.' As a matter of
/ E. 1
2 f V t Ctf EM^YOLOGY.
history w^ad'f-Kat {tTie.^udy ,of it Sprang out of the various
attempts to solve the problems of why and how living beings
come into existence.
It would be beyond the scope of this work to enter at
all fully into any account of the earlier of these inquiries
from those of Aristotle downwards ; but it may be of some
use to point out the chief steps by which in modern times
embryology has been established as a distinct branch of
knowledge.
From the very first, incubated bird's eggs, and especially
hen's eggs, owing to their abundance at all seasons, and the
ease with which they could be examined, became special
objects of study. Aristotle examined the growing chick
within, the egg, and gave the name of punctum saliens to the
' bloody palpitating point/ which marks the growing heart in
the early days of incubation. Since his time all observers
have had recourse to the hen's egg; and though it may be
urged that the highly specialised characters of the avian
type unfit it for so general a purpose as that of serving as
the foundation of embryology, the practical advantages of the
bird's egg over either the mammalian or any other ovum, are
so many, that it must always continue to be, as it has been,
a chief object of study.
From the time of Aristotle down to that of Fabricius of
Aquapendente so little progress in real observation of facts
had been made, that we find the latter anatomist (De Forma-
tione Ovi et Pulli, 1 621) describing the chick as being formed
out of the chalazae of the white of the egg; a view which
lived long afterwards, and whose influence may still be
recognized in the names 'tread' or 'treadle' which the
housewife sometimes gives to those portions of thickened
albumen.
Harvey was the first to clearly establish that the essential
part of the hen's egg, that out of which the embryo pro-
INTRODUCTION. 3
ceeded, was the cicatricula. This Fabricius had looked upon
as a blemish, a scar left by a broken peduncle. In his
Anatomical Exercises on the Generation of Animals (1651),
Harvey describes the little cicatricula as expanding under
the influence of incubation into a wider structure, which
he calls the eye of the egg ; and at the same time sepa-
rating into a colliquamentum. In this colliquamentum,
according to him, there appears, as the first rudiment of
the embryo, the heart or punctum saliens, together with
the blood-vessels. These gradually gather round them
the solid parts of the body of the chick. Harvey clearly
was of opinion that the embryo arose, by the successive
formation of parts, out of the homogeneous nearly liquid
colliquamentum. He was an early advocate of the doctrine
of epigenesis.
Notwithstanding the weight of Harvey's authority, the
doctrine of epigenesis subsequently gave way to that of
evolution, according to which the embryo pre-existed, even
though invisible, in the ovum, and the changes which took
place during incubation consisted not in a formation of
parts, but in a growth, i. e. in an expansion with concomitant
changes, of the already existing germ. Of this theory
Malpighi is frequently said to have been the founder. In
a limited sense this is true. In his letter to the Royal
Society of London, De Formatione Pulli in Ovo (1672), he
:onfesses himself compelled to admit that even in unincu-
bated eggs an embryo was present (Quare pulli stamina in
ovo pre-existere, altioremque originem nacta esse fateri
convenit). Yet he evidently struggled against such a con-
clusion, and instead of developing a consistent theory of
evolution, left the earliest stages of the embryo as too
mysterious to be profitable objects of study, and contented
himself with tracing out the events of later days. From his
descriptions it is clear that his so-called unincubated eggs
12
4 ON EMBRYOLOGY.
had under the warmth of summer already made considerable
progress in development.
The man who first logically worked out a theory of
evolution and became its most distinguished and zealous
advocate was Haller (Sur la Formation du Cceur dans le
Poulet, 1758, arid Elementa Physiologic, Liber xxix. 1766).
This great anatomist insisted that the embryo existed
even in the unincubated egg though in a rudimentary form,
and indeed invisible. He supposed that it was a vermiform
structure composed of all the essential parts of a full-grown
animal in an undeveloped state, and that the effect of incu-
bation was to educe or evolve these undeveloped organs into
an adult condition. The same views were urged with cha-
racteristic extravagance by Bonnet (Considerations sur les
corps organises, 1762).
This doctrine of evolution or prsedilineation, as it was
called at the time, was doomed to be overthrown even in
Haller 's own day.
In an inaugural dissertation entitled Theoria Generationis,
published 1759, Casper Frederick Wolff laid the foundations
of not only modern Embryology, but modern Histology. He
shewed that the cicatricula of the unincubated hen's egg con-
sisted of a congeries of particles (such as we now call cells) all
alike, or divisible into groups only, and that anything like
distinct rudiments of an embryo were wholly absent. Out of]
these particles the embryo was built up by means of a series of j
successive changes (several of which he described in detail,
especially in his work on the Formation of the Alimentary
Canal, 1768), part being added to part, and parts once formed
being modified into fresh parts. Thus the old imperfect
theory of evolution was supplanted by a view, which, under
the term of epigenesis, was in reality a more complete and
truer theory of evolution. Wolff also shewed that all the,
parts as well of plants as of animals could be conceived of
INTRODUCTION. 5
as being arrangements of these particles or cells variously
modified, and that all the phenomena of the form and
structure of living beings were to be regarded as the results
of a variable nutritive energy, to which he gave the name
vis essentialis.
Haller complained of Wolff, that he had attempted to
make a great leap instead of being contented with small on-
ward steps. Wolff's leap proved too great for his time. While
his insight into the fundamental doctrines of histology re-
mained for the most part without fruit till the next century,
so also the way he opened up in embryology was successfully
followed by no one for many years after.
In 1816 that admirable teacher Dollinger, of Wiirzburg,
induced Pander to take up the study of the incubated hen's
egg. We owe to Pander (Dissertatio Inauguralis sistens
Historiam Metamorphoseos quam Ovum Incubatum prioribus
quinque diebus subit, and Beitrdge zur Entwickelungsge-
schichte des Huhnchens im Eie) a clear and excellent descrip-
tion of many of the changes which take place during the
early days of incubation. It was he who introduced the term
blastoderm. He too first drew attention to the distinction of
the three layers, serous, mucous, and vascular. But his
greatest merit perhaps consisted in the fact of his studies
having been the exciting cause of those of Von Baer.
Coming to Wiirzburg to study under Dollinger, and finding
Pander busily engaged in his embryological work, Von Baer
enthusiastically took up the same subject, and thenceforward
devoted the greater part of his life to it.
Of the results of his labours, which are embodied in his
Entwickelungsgeschichte der Thiere, 1828, 1837, this simply
may be said. Von Baer found the true line of inquiry already
marked out by Wolff. He followed up that line so sedulously
and with such success, that nearly all the work which has
been done since his day up to the present time, in Vertebrate
6 ON EMBRYOLOGY*
Embryology, may be regarded as little more than an ex-
tension, with corrections, of his observations. Were it de-
sirable to re-publish Von Baer's work, the corrections and
expansions of matters of fact necessary to bring it up to the
present time, as the phrase goes, would, with some few
exceptions, be of minor importance, though they might be
many. The theoretical considerations embodied in his
Scholia through which he interprets the morphological sig-
nificance of embryological facts are of great and lasting
importance, though they need some modifications in order to
bring them into harmony with the theory of natural selection.
Since Von Baer's time, the advances made in Vertebrate Em-
bryology, through the elaborate work of Remak, the labours of
Eathke, Allen Thomson and others, the admirable lectures of
Kolliker, and the researches of more recent inquirers, though
many and varied, cannot be said to constitute any epochs in
the history of the subject, such as that which was marked by
Von Baer, and before him by Wolff. We may perhaps make
an exception in favour of the discovery by Purkinje, of the
germinal vesicle in the fowl's ovarian ovum (1825). This led
to Von Baer's discovery of the mammalian ovum (1827), which
first rendered possible a consistent view of mammalian gene-
ration.
The study of invertebrate embryology has, 'on the other
hand, during the last few years produced the most striking
results.
In the following pages we propose to follow in the path
thus marked out by the history of the subject. We begin
with the chick as being the animal which has been most
studied, and the study of which is easiest, and most fruitful
for the beginner. The first part accordingly will be devoted
to a description of the changes undergone by an incubated
hen's egg, especially during the early days of incubation. We
shall endeavour to explain, with such details as are necessary,
INTRODUCTION. 7
the manner in which the embryo is formed, and the way in
which the rudiments of the most important organs of the
chick arise. We shall follow a chronological order, tracing out
the changes day by day (or with even shorter periods), during
the first few days. We are convinced that this method
(adopted by Von Baer) is on the whole the one which most
commends itself to the learner. It has of course its disad-
vantages ; and in several instances we have found it desirable
when describing, at its appropriate date, the most striking
phase in the development of an organ, at once to follow up
the subsequent history, instead of giving it piecemeal after-
wards. But the general advantages of the chronological
method, especially when the reading of such a book as this
is rendered really useful by an Accompanying actual exami-
nation of incubated eggs, are so great that they far outweigh
the evil of any such slight irregularities. After tracing out
the history of the several organs, no farther than is necessary
to give a clear idea of the general course of events in each
case, we propose to treat the changes and incidents of the
latter days of incubation with great brevity, not attempting
any special account of avian development, except in the case
of the skull. And even this will be treated summarily. The
First Part will therefore really be an introduction to the
general facts of vertebrate embryology, the chick being taken
as an example.
In the Second Part we purpose to consider the embryonic
histories of other vertebrates, in so far as these differ from that
of the bird ; and then to treat of the development of special
organs in a more complete manner.
The Third Part will be devoted to an exposition of the
main facts of invertebrate embryology, and to the discussion
of general morphological considerations.
8 ON EMBRYOLOGY.
The reader will scarcely fail to notice that the First Part
especially is entirely confined to a simple description of
observed facts, no attempt whatever being made to interpret
their meanings. We have purposely pursued this course,
because any interpretation of the facts of the bird's develop-
ment is impossible, or at least illusory, till the history of other
animals, vertebrate and invertebrate, has been studied.
When all the facts are before him the reader will be in a
position to judge of the interpretations offered.
PART I.
THE HISTORY OF THE CHICK.
CHAPTER I.
THE STRUCTURE OF THE HEN'S EGG, AND THE CHANGES
WHICH TAKE PLACE UP TO THE BEGINNING OF INCUBATION.
1. IN a hen's egg quite newly laid we meet with the
following structures. Most external is the shell (Fig. 1, .),
composed of an organic basis, impregnated with calcic salts.
It is sufficiently porous to allow of the interchange of gases
between its interior and the external air, and thus the
chemical processes of respiration, feeble at first, but gradually
increasing in intensity, are carried on during the whole
period of incubation.
According to Nathusius, Zeitsch. f. Wiss. Zool. Vol. xvm. p. 225 270,
XIX. 322348, XX. 106 120, xxi. 330 355, the egg-shell of birds consists
of an outer thinner and an inner thicker layer. The outer layer varies con-
siderably in its consistency in different species. It is soft and pliant in the hen,
but in many other birds, as for instance the ostrich, is hard and friable. It is
frequently striated both vertically and transversely. Pigment when present is
confined to this layer. The inner layer is thicker ; and its internal surface is
marked with rounded processes more or less separated from one another, whose
blunt extremities are sunk into the shell-membrane. The presence of these pro-
cesses must be considered as universal amongst birds. Vertical sections shew
that this layer is composed of alternating horizontal laminae of transparent and
opaque material, the opaque laminae being composed of exceedingly minute par-
ticles of an organic nature imbedded in a matrix impregnated with calcic salts.
Both layers of the shell are pierced by vertical canals, which are simple in
Carinate but ramified in Katite birds. These canals open freely on the exterior
surface and also on the interior surface in the pits between the blunt processes
of the inner layer. It is probable that the outer openings of these canals
become closed by the presence of moisture, so that when the shell is wet neither
air nor water can pass through it. If the shell is dry, air will penetrate easily ;
and if the upper layer with the free ends of the tubes be rubbed off, both water
and air will pass through it without difficulty. In eggs with coloured shells
the colouring matter frequently passes into the canals.
12 THE HEN'S EGG. [CHAP.
2. Lining the shell, is the shell-membrane, which is
double, being made up of two layers; an outer thicker
(Fig. 1, s. m.), and an inner thinner one (i. s. m.). Both of
these layers consist of several laminae of felted fibres of l
various sizes, intermediate in nature between connective
and elastic fibres.
FIG. i.
ch.l.
DIAGRAMMATIC SECTION OF AN UNTNCUBATED FOWL'S EGG
(modified from Allen Thomson).
bl. blastoderm, w. y. white yolk. This consists of a central flask-shaped
mass and a number of layers concentrically arranged around this.
y. y. yellow yolk. v. t. vitelline membrane, x. layer of more fluid
albumen immediately surrounding the yolk. w. albumen consisting of
alternate denser and more fluid layers, ch. I. chalaza. a. ch. air-chamber
at the broad end of the egg. This chamber is merely a space left between
the two layers of the shell-membrane, i. s. m. internal layer of shell-
membrane, s. m. external layer of shell-membrane, s. shell.
Over the greater part of the egg the two layers of the
shell-membrane remain permanently in close apposition
to each other; but at the broad end they tend to separate,
and thus to develope between them a space into which air
finds its way. This air-chamber, as it is called, is not to be
found in perfectly fresh eggs, but makes its appearance in
I.] THE WHITE OF THE EGG. 13
eggs which have been kept for some time, whether incubated
or not, and gradually increases in size, as the white of the
egg shrinks in bulk by evaporation.
3. Immediately beneath the shell-membrane is the white
of the egg or albumen (Fig. 1, w.\ which is, chemically
speaking, a mixture of various forms of proteid material,
with fatty, extractive, and saline bodies.
Its average composition may be taken as
1 2*0 p. c. proteid matter,
I "5 p. c. fat and extractives,
5 p. c. saline matter, chiefly sodic and potassic chlorides, with phos-
phates and sulphates,
86'o p. c. water.
The white of the egg when boiled shews in section
alternate concentric layers of a transparent and of a finely
^granular opaque material. In the natural condition, the layers
corresponding to these opaque layers are composed of more
fluid albumen, while those corresponding to the transparent
layers are less fluid, and consist of networks of fibres, con-
taining fluid in their meshes. The outer part of the white,
especially in eggs which are not perfectly fresh, is more fluid
than that nearer the yolk. The innermost layer, however,
immediately surrounding the yolk (Fig. 1, a?.), is of the more
fluid finely granular kind.
In eggs which have been hardened a spiral arrangement
of the white may be observed, and it is possible to tear off
laminae in a spiral direction from left to right, from the
broad to the narrow end of the egg.
Two twisted cords called the Chalazce (Fig. 1, ch. L), com-
posed of coiled membranous layers of the less fluid albumen,
run from the two extremities of the egg to the opposite
portions of the yolk. Their inner extremities expand and
merge into the layer of denser albumen surrounding the
fluid layer next the yolk. Their outer extremities are free,
and do not quite reach the outer layer of the white. Thus
they cannot serve to suspend the yolk, although they may
help to keep it in position, by acting as elastic pads. The
interior of each chalaza presents the appearance of a suc-
cession of opaque white knots ; hence the name chalazse,
grandines (hailstones).
4. The yolk is enclosed in the vitelline membrane
(Fig. 1, v. .), a transparent somewhat elastic membrane easily
14 THE HEN'S EGG. [CHAP.
thrown into creases and wrinkles. It might almost be called
structureless, but under a high power a fine fibrillation is
visible, and a transverse section has a dotted or punctated
appearance ; it is probably therefore composed of fibres.
Its affinities are with elastic rather than connective tissue.
The vitelline membrane of most vertebrates is perforated by fine pores.
These are largest in osseous fishes and much finer in mammals; they have not
been found in the vitelline membrane of birds.
5. The whole space within the vitelline membrane is
occupied by the yolk. To the naked eye this appears toler-
ably uniform throughout, except at one particular point of
its surface, at which may be seen, lying immediately under
the vitelline membrane, a small white disc, about 4 mm. in
diameter. This is the blastoderm, or cicatricula.
A tolerably typical cicatricula in a fecundated egg will
shew an outer white rim of some little breadth, and within
that a circular transparent area, in the centre of which,
again, there is an opacity, varying in appearance, sometimes
uniform, and sometimes dotted.
The disc is always found to be uppermost whatever be
the position of the egg, provided there is no restraint to
the rotation of the yolk. The explanation of this is to be
sought for in the lighter specific gravity of that portion of
the yolk which is in the neighbourhood of the disc, and the
phenomenon is not in any way due to the action of the
chalazae.
A section of the yolk of a hard-boiled egg will shew that
it is not perfectly uniform throughout, but that there is a
portion of it having the form of a flask, with a funnel-
shaped neck, which, when the egg is boiled, does not become
so solid as the rest of the yolk, but remains more or less fluid.
The expanded neck of this flask-shaped space is situated
immediately underneath the disc, while its bulbous enlarge-
ment is about the middle of the yolk. We shall return to
it directly.
6. The great mass of the yolk is composed of what is
known as the yellow yolk (Fig. 1, y. y.). This consists of
spheres (Fig. 2, A.} of from 25/4 to 100/x 1 in diameter, never
containing a nucleus, but filled with numerous minute highly
refractive granules ; these spheres are very delicate and easily
= 'ooi mm.
I.] THE WHITE YOLK. 15
destroyed by crushing. When boiled or otherwise hardened
in situ, they assume a polyhedral form, from mutual pressure.
The granules they contain seem to be of an albuminous
nature, as they are insoluble in ether or alcohol.
Chemically speaking the yolk is characterized by the presence in large
quantities of a proteid matter, having many affinities with globulin, and called
vitellin. This exists in peculiar association with the remarkable body Lecithin.
(Compare Hoppe-Seyler, Hdb. Phys. Chem. Anal.) Other fatty bodies,
colouring matters, extractives (and, according to Dareste, starch in small quan-
tities), &c. are also present. Miescher (Hoppe-Seyler, Chem. Untersuch. p. 502)
states that a considerable quantity of nuclein may be obtained from the yolk,
probably from the spherules of the white yolk.
FIG. 2.
A. Yellow yolk-sphere filled with fine granules. The outline of the sphere
has been rendered too bold.
B. White yolk-spheres and spherules of various sizes and presenting different
appearances. (It is very difficult in a woodcut to give a satisfactory repre-
sentation of these peculiar structures.)
7. The yellow yolk thus forming the great mass of the
entire yolk is clothed externally by a thin layer of a different
material, known as the white yolk, which at the edge of
the blastoderm passes underneath the disc, and becoming
thicker at this spot forms, as it were, a bed on which the
blastoderm rests. Immediately under the middle of the
blastoderm this bed of white yolk is connected, by a narrow
neck, with a central mass of similar material, lying in the
middle of the yolk (Fig. 1, w. y.}. When boiled, or otherwise
hardened, the white yolk does not become so solid as the
yellow yolk; hence the appearances to be seen in sections
of the hardened yolk. The upper expanded extremity of
this neck of white yolk is generally known as the "nucleus of
Pander."
Concentric to the outer enveloping layer of white yolk
there are within the yolk other inner layers of the same
substance, which cause sections of the hardened yolk to
16 THE HEN'S EGG. [CHAP.
appear to be composed of alternate concentric thicker laminae
of darker (yellow) yolk, and thinner laminae of lighter (white)
yolk (Fig. 1, w, y.).
8. The microscopical characters of the white yolk are
very different from those of the yellow yolk. It is composed
of spheres (Fig. 2, S.) for the most part smaller than those of
the yellow yolk (4/x 75/z,), with a highly refractive nucleus-
like body often as small as l/^ in the interior of each; and
also of larger spheres, each of which contains a number of
spherules, similar to the smaller spheres ; these latter appear-
ing to have passed into the larger spheres, by a process of
inclusion.
There has been a considerable amount of controversy as to whether these
elements possess a membrane ; there is little doubt however that there is no
membrane present.
It has also been disputed as to whether they should be considered as true
cells or not. If by definition a cell must contain a nucleus, they can hardly
be considered as such, since the characters of the highly refractive bodies con-
tained in them have nothing in common with nuclei. We shall give later on
reasons for thinking that they may however, as a result of incubation, become
veritable cells.
Another feature of the white yolk, according to His, is
that in the region of the blastoderm it contains numerous
large vacuoles rilled with fluid ; they are sufficiently large to
be seen with the naked eye, but do not seem to be present in
the ripe ovarian ovum.
9. It is now necessary to return to the blastoderm. In
this, as we have already said, the naked eye can distinguish
an opaque white rim surrounding a more transparent central
area, in the middle of which again is a white spot of variable
appearance. In an unfecundated cicatricula the white disc
is simply marked with a number of irregular clear spaces,
there being no proper division into a transparent centre and
an opaque rim.
The opaque rim is the commencement of what we shall
henceforward speak of as the area opaca ; the central trans-
parent portion is in the same way the beginning of the area
pellucida. At this stage the distinction between these two
areas depends entirely on the disposition of the white yolk
beneath them, for the blastoderm when lifted up from the
white yolk on which it rests appears uniform throughout.
In the part corresponding to the area opaca the blastoderm
rests immediately on the white yolk, which here forms a
I.] THE BLASTODERM. 17
somewhat raised ring, often spoken of as the germinal wall ;
underneath the area pellucida is a shallow space containing
a nearly clear fluid, to the presence of which the central
transparency seems to be due. The white spot in the middle
of the area pellucida appears to be the nucleus of Pander
shining through.
Vertical sections of the blastoderm shew that it is formed
of two layers. The upper of these two layers is com-
posed, see Fig. 3, ep, of a single layer of cells, with their
long axes arranged vertically, adhering together so as to
form a distinct membrane, the edge of which rests upon
the white yolk. After staining with silver nitrate, this
membrane viewed from above shews a mosaic of uniform
polygonal cells.
Each cell is composed of granular protoplasm filled with
highly refractive globules; in most of the cells an oval
nucleus may be distinguished, and is most probably present
in all. They are of a uniform size (about 9 /JL) over the
opaque and the pellucid areas.
The under layer (Fig. 8, Z), is composed of cells which
vary considerably in diameter ; but even the smaller cells of
this layer are larger than the cells of the upper layer.
They are spherical, and so filled with granules and highly
refractive globules, that a nucleus can rarely be seen in them:
in the larger cells these globules contain a highly refractive
body very similar to that present in the white yolk spheres,
from the smaller kinds of which indeed they are scarcely
distinguishable.
The cells of this layer do not form a distinct membrane
like the cells of the upper layer, but lie as a somewhat
irregular network of cells between the upper layer and the
bed of white yolk on which the blastoderm rests. The
lowest are generally the largest; in addition we find a
few still larger cells generally separated by a small interval
from the remainder of the cells of the lower layer, and
resting directly upon the white yolk (Fig. 3, 6). These are
frequently spoken of as formative cells; they are however
similar in character and indeed connected by gradations
with the larger cells of the lower layer. Their mode of
formation during segmentation will be subsequently de-
scribed. '
E. 2
18
THE HEN'6 EGG.
[CHAP.
FIG. 3.
SECTION OF A BLASTODEBM OF A FOWL'S EGG AT THE
<foMMENCEMENT OF INCUBATION.
The thin but complete upper layer ep composed of
columnar cells rests on the incomplete lower layer /,
composed of larger and more granular bodies. The lower
layer is thicker in some places than in others, and is
especially thick at the periphery. The line below the
under layer marks the upper surface of the white yolk*
The larger so-called formative cells are seen at 6, lying
on the white yolk. The figure does not take in quite
the whole breadth of the blastoderm ; but the reader must
understand that both to the right hand and to the left ep
is continued farther than /, so that at the extreme edge
it rests directly on the white }olk.
Over nearly the whole of the blastoderm
the upper layer rests on the under layer.
At the circumference however the upper
layer stretches for a short distance beyond
the under layer, and here consequently rests
directly on the white yolk, and forms that
part of the blastoderm known as the area
opaca*
10. To recapitulate : In the normal
Unincubated hen's egg we recognize the
blastoderm, consisting of a complete upper
layer of smaller nucleated granular cells and
a more or less incomplete under layer of
larger cells, filled with larger granules ; in
these lower cells nuclei are rarely visible.
The thin flat disc so formed rests, at the
uppermost part of the entire yolk, on a bed
of white yolk so disposed as to give rise to
the appearance in the blastodermic disc it-
self of an area opaca and an area pellucida.
The great mass of the entire yolk consists of
the so-called yellow yolk composed of gra-
nular spheres. The white yolk is composed
of smaller spheres of peculiar structure, and
exists, in small part, as a thin coating around,
and as thin concentric laminae in the sub-
** * stance of the yellow yolk, but chiefly in the
form of a flask-shaped mass in the interior
of the yolk, the upper somewhat expanded top of the neck
I.] THE OVARIAN OVUM. 19
of which forms the bed on which the blastoderm rests. The
whole yolk is invested with the vitelline membrane, this
again with the white ; and the whole is covered with two
shell-membranes and a shell.
11. Such an egg has however undergone most important
changes while still within the body of the hen ; and in
order to understand the nature of the structures which have
just been described, it will be necessary to trace briefly the
history of the egg from the stage when it exists as a so-called
ripe ovarian ovum in the ovary of a hen up to the time when
it is laid.
If one of the largest capsules of the ovary of a hen which
is laying regularly be opened, it will be found to contain a
nearly spherical (or more correctly, ellipsoidal with but
slightly unequal axes) yellow body enclosed in a delicate
membrane. This is the ovarian ovum or egg. Examined
with care the ovum, which is tolerably uniform in appearance,
will be found to be marked at one spot (generally lacing the
stalk of the capsule and forming the pole of the shorter axis
of the ovum) by a small disc differing in appearance from
the rest of the ovum. This disc is known as the germinal
disc or discus proligerus. It consists of a lenticular mass of
w.y.
SECTION THROUGH THE GERMINAL Disc OF THE RIPE OVARIAN OVUM OP A
FOWL WHILE YET ENCLOSED IN ITS CAPSULE.
a. Connective-tissue capsule of the ovum. b. epithelium of the capsule, at the
surface of which nearest the ovum lies the vitelline membrane, c. granular
material of the germinal disc, which becomes converted into the blastoderm.
(This is not very well represented in the woodcut. In sections which
have been hardened in chromic acid it consists of fine granules.) w. y,
white yolk, which passes insensibly into the fine granular material of the
disc, x, germinal vesicle enclosed in a distinct membrane, but shrivelled
up by the action of the chromic acid. The material enclosed in the
membrane of the vesicle is in the hardened specimens finely granular,
y, space originally completely filled up by the germinal vesicle, before the
latter was shrivelled up by the action of the chromic acid.
22
20 THE HEN'S EGG. [CHAP.
protoplasm (Fig. 4, c), imbedded in which is a highly refrac-
tive globular or ellipsoidal body (Fig. 4, x), about 310/u, in
diameter, called the germinal vesicle, in the interior of which
again is a small body, the germinal spot.
The rest of the ovum is known as the yolk. This consists
of two elements, the white yolk- and the yellow yolk-spheres,
which are distributed respectively very much in the same
way as in the laid egg, the yellow yolk forming the mass of
the ovum, and the white yolk being gathered underneath and
around the disc (Fig. 4, w. y\ and also forming a flask-
shaped mass in the interior of the ovum. The delicate
membrane surrounding the whole is the vitelline membrane.
Oellacher's (Untersuchung uber die Furchung und BlatterUldung in Huh-
nereie. Studien aus dem Institute fur experimental pathologic in Wien aus
dem Jahre 1869, pt. l) account of the ovarian ovum differs considerably
from that given above. He finds in the neighbourhood of the blastoderm
a finely granular material, within which lies a body appearing circular when
viewed from above, but having in section a somewhat quadrilateral shape;
its side-walls, however, are curved, with their convexity turned inwards. At
the bottom of it lies an oval cavity with doubly contoured walls, and at its
upper surface placed somewhat excentrically a semicircular space filled with
clear material.
Oellacher believes that the quadrilateral body which he thus describes is
the germinal vesicle which has commenced to undergo a retrogressive meta-
morphosis. For the further stages in the metamorphosis, and for further par-
ticulars, vide Section 13. The circular hole beneath the vesicle is probably
merely filled with fluid and is due to the contractions of the germ.
12. When the ovarian ovum is ripe and about to be dis-
charged from the ovary, its capsule is clasped by the dilated
termination of the oviduct. The capsule then bursts, and
,the ovum escapes into the oviduct, its longer axis corre-
sponding with the long axis of the oviduct, the germinal disc
therefore being to one side. At the time of the bursting of
the capsule the germinal vesicle disappears.
In describing the changes which take place in the
oviduct, it will be convenient, following the order previously
adopted, to treat first of all of the formation of the accessory
parts of the egg. These are secreted by the glandular walls
of the oviduct. This organ therefore requires some descrip-
tion. It may be said to consist of four parts ; 1st. The
dilated proximal extremity. 2nd. A long tubular portion,
opening by a narrow neck or isthmus into the 3rd portion,
which is much dilated, and has been called the uterus; the
ith part is somewhat narrow, and leads from the uterus
I.] DESCENT OF THE OVUM. 21
into the cloaca. The whole of the mucous membrane lining
the oviduct is largely ciliated.
The accessory parts of the egg are entirely formed in the
2nd and 3rd portions. The layer of albumen which imme-
diately surrounds the yolk is first deposited; the chalazae are
next formed. Their spiral character arid the less distinctly
marked spiral arrangement of the whole albumen is brought
about by the motion of the egg along the spiral ridges into
which the interior of the second or tubular portion of the
oviduct is thrown. The spirals of the two chalazae are in
different directions. This is probably produced by their
peripheral ends remaining fixed while the yolk to which
their central ends are attached is caused to rotate by the
contractions of the oviduct. During the formation of the
chalazae the rest of the albumen is also deposited; and
finally the shell-membrane is formed in the narrow neck
of the 2nd portion, by the fibrillation of the most external
layer of albumen. The egg passes through the 2nd portion
in little more than 3 hours. In the 3rd portion the shell is
formed. The mucous membrane of this part is raised into
numerous flattened folds, like la,rge villi, containing follicu-
lar glands. From these a thick white fluid is poured out,
which ' soon forms a kind of covering to the egg, in which
the inorganic particles are deposited. In this portion of the
oviduct the egg remains from 12 to 18 hours, during which
time the shell acquires its normal consistency. At the time
of laying it is expelled from the uterus by violent muscular
contractions, and passes with its narrow end downwards
along the remainder of the oviduct, to reach the exterior.
13. We have now to trace out the changes which take
place in the germinal disc, during the passage of the egg
down the oviduct.
By the time when the egg becomes clasped by the expanded extremity of
the oviduct the germinal vesicle has, according to Oellacher (loc cit. and also
Archiv. fur Micr. Anat. Vol. vin. 1872. p. 18), undergone still further
retrogressive changes. It has now become very much flattened and closely
applied to the vitelline membrane. Both this and former stages, if we may
judge from the analogy of osseous fishes, are preparatory to the whole germinal
vesicle being bodily ejected from the germinal disc. For further particulars
vide Oellacher, Archiv. fur Micr. Anat. Vol. vill. pp. i 26.
Impregnation occurs in the upper portion of the oviduct ;
the spermatozoa being found actively moving in 'a fluid which
is there contained.
22 THE HEN'S EGG. [CHAP.
It is not certain whether impregnation takes place previous to the deposition
of the albumen, or whether the spermatozoa bore their way through the albumen.
The former would appear to be the more probable view, though the fact that
Oellacher has found spermatozoa in the albumen, speaks in favour of their being
involved in the depositing albumen, and so being brought in contact with tiie
blastoderm.
According to Coste, Histoire du developpement des corps organizes, the access
of the cock to the hen once in seven days is sufficient.
We have no positive evidence that the spermatozoa make
their way through the vitelline membrane arid so gain access
to the germinal disc; but, as will be seen in a later part of
this work, analogy renders such an event probable.
14. At about the time when the shell is being formed
round the egg, the germinal disc undergoes a remarkable
change, known as segmentation. We shall have occasion to
treat more fully of the nature of segmentation when we
come to consider the amphibian ovum in which the various
steps of the process may be more easily and satisfactorily
traced. Meanwhile, inasmuch as the segmentation of the
SURFACE VIEWS OP THE EARLY STAGES OF THE SEGMENTATION IN A
FOWL'S EGG. (After Coste.)
A represents the earliest stage. The first furrow (b) has begun to make its
appearance in the centre of the germinal disc, whose periphery is marked by
the line a. In , the first furrow is completed right across the disc, and a
second similar furrow at nearly right angles to the first has appeared. The disc
thus becomes divided somewhat irregularly into quadrants by four (half) furrows.
In a later stage (C) the meridian furrows b have increased in number, from
four, as in Z?, to nine, and cross furrows have also made their appearance. The
disc is thus cut up into small central (c) and larger peripheral (d) segments.
Several new cross furrows are seen just beginning, as ex. gr. close to the end of
the line of reference d.
I.]
SEGMENTATION.
23
germinal disc of a hen's egg differs materially from the
segmentation of the entire ovum of an amphibian, the former
may briefly be described here.
Viewed from above, a furrow is seen to make its appear-
ance, running across the germinal disc and dividing it into
two halves (Fig. 5, A). This primary furrow is succeeded
by a second at right angles to itself. The surface thus
becomes divided into four segments or quadrants (Fig. 5, B).
Each of these is again bisected by radiating furrows, and
thus the number of segments is increased from four to
eight (it may be seven or nine). The central portion of
each segment is then, by a cross furrow, cut off from the
peripheral portion, giving rise to the appearance of a number
of central smaller segments, surrounded by more external
elongated segments (Fig. 5, C).
Division of the segments now proceeds rapidly by means
of furrows running apparently in all directions. And it is
FIG. 6.
SURFACE VIEW OF THE GERMINAL Disc OF A HEN'S EGG DURING THE LATER
STAGES OF SEGMENTATION., (Chromic Acid Preparation.)
At c in the centre of the disc the segmentation masses are very small and
numerous. At 6. nearer the edge, they are larger and fewer; while those at
the extreme margin a are largest and fewest of all. It will be noticed that the
radiating furrows marking off the segments a do riot as yet reach to the extreme
margin e of the disc.
The drawing is completed in one quadrant only; it will of course be under-
stood that the whole circle ought to be filled up in a precisely similar manner.
THE HEN'S EGG.
[CHAP.
important to note that the central segments divide more
rapidly than the peripheral, and consequently become at once
smaller and more numerous (Fig. 6).
Meanwhile sections of the hardened blastoderm teach us
that segmentation is not confined to the surface, but extends
through the mass of the blastoderm; they shew us moreover
that division takes place by means of not only vertical, but
also horizontal furrows, i.e. furrows parallel to the surface of
the disc (Fig. 7).
FIG. 7.
SECTION OF THE GERMINAL Disc OF A FOWL DURING THE LATER STAGES
OF SEGMENTATION.
The section, which represents rather more than half the breadth of the
blastoderm (the middle line being shewn at c), shews that the upper and central
parts of the disc segment faster than those below and towards the periphery.
At the periphery the segments are still very large. One of the larger segments
is shewn at a. In the majority of segments a nucleus can be seen ; and it seems
probable that a nucleus is present in them all. Most of the segments are filled
with highly refracting spherules^ but these are more numerous in some cells
(especially the larger cells near the yolk) than in others. In the central part of
the blastoderm the upper cells have commenced to form a distinct layer. No
segmentation-cavity is present.
a. large peripheral cell. b. larger cells of the lower parts of the blastoderm.
c. middle line of blastoderm, e. edge of the blastoderm adjoining the white
yolk. w. white yolk.
In this way, by repeated division, or segmentation, the
original germinal disc is cut up into a large number of small
rounded masses of protoplasm, which are smallest in the
centre, and increase in size towards the periphery. The
segments lying uppermost are moreover smaller than those
beneath, and thus the establishment of the two layers of
the blastoderm is foreshadowed.
I.] SEGMENTATION. 25
According to Oellacher, Studien aus dem Ins. f. Exper. Pathol. Vien. 1869,
p. I, sections taken through the centre of the germinal d ; sc at the beginning
of segmentation shew a somewhat uneven vertical furrow, ending below in a
small triangular space, where it joins a nearly horizontal furrow which meets
the surface of the egg at some little distance on either side of the vertical
furrow. It seems certain that these first-formed furrows do not include the
\vhole of the germinal disc, whose limits at this st:<ge are however uncertain.
In the later stages of segmentation not only do the first-formed segments
become further divided, but segmentation also extends into the remainder of
the germinal disc. Goette, Arckiv. Micr. Anat. x. 145, indeed maintains that
segmentation (at a later period) even involves material which is undoubtedly
white yolk. He describes nuclei as making their appearance in the upper
surface of the bed of white yolk, and the substance round them as rising up in
the form of papilhe, which are subsequently constricted off and set free as
supplementary segmentation masses. It is these, according to him, which give
rise to the formative cells spoken of in the next paragraph. He states that they
continue to be formed long after the commencement of incubation. We shall
return to this subject, when we come to discuss more fully the nature of the
process of segmentation, in describing the ova of other classes of vertebrates.
Between the segmented germinal disc, which we may
now call the blastoderm, and the bed of white yolk on which
it rests, a space containing fluid makes its appearance. This,
gradually increasing in all dimensions, may be called the
segmentation-cavity.
15. As development proceeds, segmentation reaches its
limits in the centre, but continues at the periphery, and
thus eventually the masses at the periphery become of the
same size as those in the centre.
The distinction however between an upper and a lower
layer becomes more and more obvious.
The masses of the upper layer arrange themselves, side
by side, with their long axes vertical ; their nuclei become
very distinct. In fact they form a membrane of columnar
nucleated cells.
The masses of the lower layer, remaining larger than
those of the upper layer, continue markedly granular and
round, and form rather a close irregular network than a
distinct membrane. In them nuclei are either wholly absent
or at least not readily visible.
It seems more probable that the nucleus is hidden than that it is really
absent. In the earliest stages of segmentation which we have examined when
the segments were still few in number, a very large proportion of both great
and small segments contained large well-formed nuclei. These nucleated
segments, which were found in both the superficial and deeper portions of the
disc, were invariably those in which the granules were for some reason or other
few and fine; in fact, wherever the granules were not sufficiently numerous to
render the body of the segment too opaque, there a nucleus could be detected.
26 THE HEN'S EGG. [CHAP. i.
We were thus led to the conclusion that a nucleus really existed in all. It is
of course quite possible that the clearer nucleated masses eventually come to
the surface and leave the more granular and opaque masses to form the lower
layer; but it is much more likely that they do not, and that the granular con-
dition of the cells of the lower layer of the fully formed blastoderm is on the
one hand the result of their being in immediate contact with the excessively
granular white yolk-cells, and on the other the cause of their nuclei not being
seen. We have a somewhat analogous case in the invisibility of the nucleus in
the early stages of the amphibian blood-corpuscle.
At the time when the segmentation-spheres in the centre
are smaller than those at the periphery, and those above
are also smaller than those below, a few large spherical
masses begin to separate from the remainder (or to arise
by a continued process of segmentation from the bed of
white yolk), and to rest directly on the white yolk, at the
bottom of the shallow segmentation-cavity. They contain
either numerous small nucleated spherules, or fine granules ;
the spherules precisely resembling the smaller spheres of
white yolk. These loose spherical masses are the formative
cells already spoken of.
Thus the original germinal disc of the ovarian ovum,
its germinal vesicle having disappeared, becomes, by the
process of segmentation, converted into a blastoderm such as
is met with in the egg when laid, into an upper layer of
columnar nucleated cells, and into a lower layer of irregularly
disposed rounded masses which have not yet definitely ac-
quired the character of cells, accompanied by a few stray
" formative " cells lying loose in the segmentation-cavity.
CHAPTER II.
A BRIEF SUMMARY OF THE WHOLE HISTORY OF INCUBATION.
1. STEP by step the simple two-layered blastoderm de-
scribed in the previous chapter is converted into the complex
organism of the chick. The details of the many changes
through which this end is reached will perhaps be rendered
more intelligible if we prefix to the special history of them
a brief summary of the general course of events from the
beginning to the end of incubation.
In the first place, it is to be borne in mind that the
embryo itself is formed in the area pellucida, and in the
area pellucida alone. The area opaca in no part enters
directly into the body of the chick; the structures to
which it gives rise are to be regarded as appendages, which
sooner or later disappear.
2. The blastoderm at starting consists of two layers.
Very soon a third layer makes its appearance between the
other two. These three layers, the establishment of which
is a fact of fundamental importance in the history of the
embryo, are called respectively the upper, middle and lower
layers, or epiblast, mesoblast and hypoblast.
This triple division corresponds roughly, though not exactly, to the old
division into serous, vascular and mucous layers.
3. The blastoderm which at first, as we have seen, lies
like a watch-glass over the segmentation-cavity, its margin
resting on the circular germinal wall of white yolk, spreads,
as a thin circular sheet, over the yolk, immediately under
the vitelline membrane. Increasing uniformly at all points
of its circumference, the blastodermic expansion covers more
and more of the yolk, and at last, reaching its opposite pole
28 PRELIMINARY ACCOUNT. [CHAP.
completely envelopes it. Thus the whole yolk, instead of
being enclosed as formerly by the vitelline membrane alone,
comes to be also enclosed in a bag formed by the blastoderm.
It is not however until quite a late period that the
complete closing in at the opposite pole takes place, so
that the extension of the blastoderm must be thought of as
going on during nearly the whole period of incubation.
Both the area opaca and the area pellucida share in
this enlargement, but the area opaca increases much more
rapidly than the area pellucida, and plays the principal
part in encompassing the yolk.
4. The mesoblast, in that part of the area opaca which
is nearest to the area pellucida, becomes the seat of peculiar
changes, which result in the formation of blood-vessels.
Hence this part of the area opaca is called the vascular area.
5. The embryo itself may be said to be formed by a
folding off of the central portion of the area pellucida from
the rest of the blastoderm. At first the area pellucida is
quite flat, or, inasmuch as it forms part of the circumference
of the yolk, slightly but uniformly curved. Very soon, how-
ever, there appears at a certain spot a semilunar groove, at
first small, but gradually increasing in depth and extent ; this
groove, which is represented in section in the diagram (Fig.
8, A) 9 breaks the uniformity of the level of the area pellucida.
It may be spoken of as a tucking in of a small portion of the
blastoderm in the form of a crescent. When viewed from
above, it presents itself as a curved line (the hinder of the
two concentric curved lines in front of A in Fig. 11), which
marks the hind margin of the groove, the depression itself
being hidden. In a vertical longitudinal section carried
through the middle line, we may recognize the following
parts (Fig. 8, A, or on a larger scale Fig. 9, which also
shews details which need not be considered now). Beginning
at what will become the posterior extremity of the embryo,
(the left-hand side of the figure in each case), and following
the surface of the blastoderm forwards (to the right in the
figures), the level is maintained for some distance, and then
there is a sudden descent, the blastoderm bending round and
pursuing a precisely opposite direction to its previous one,
running backwards instead of forwards, for some distance.
It soon however turns round again, and once more running
II.]
THE HEAD-FOLD.
FIG. 8.
29
Fig. 8, A to N forms a series of purely diagrammatic representations in-
troduced to facilitate the comprehension of the manner in which the body of
the embryo is formed, and of the various relations of the yolk-sac, amnion
and allantois.
In all vt is the vitelline membrane, placed, for convenience sake, at some
distance from its contents, and represented as persisting in the later stages ; in
the actual egg it is in direct contact with the blastoderm (or yolk), and early
ceases to have a separate existence. In all e indicates the embryo, pp the
general pleuro-peritoneal space, af the folds of the amnion, a the amnion proper,
ae or ac the cavity holding the liquor amnii, al the allantois, a' the alimentary
canal, y or ys the yolk or yolk-sac.
J., which may be considered as a vertical section taken longitudinally along
the axis of the embryo, represents the relations of the parts of the egg at
the time of the first appearance of the head-fold, seen on the right-hand side
of the blastoderm e. The blastoderm is spreading both behind (to the left
hand in the figure), and in front (to right hand) of the head -fold, its limits
being indicated by the shading and thickening for a certain distance of the
margin of the yolk y. As yet there is no fold on the left side of e correspond-
ing to the head-fold on the right, while therefore the front limit of the embryo,
as distinguished from the blastoderm, is marked out by the head-fold, there
is at present no tail-fold, and therefore no hind limit.
B is a vertical transverse section of the same period drawn for convenience
sake on a larger scale (it should have been made flatter and less curved). It
shews that the blastoderm (vertically shaded) is extending laterally as well as
fore and aft, in fact in all directions; but there are no lateral folds, and there-
30
PRELIMINARY ACCOUNT.
[CHAP.
fore no lateral limits to the body of the embryo as distinguished from the
blastoderm.
Incidentally it shews the formation of the medullary groove by the rising
up of the laminae dorsales. Beneath the section of the groove is seen the
rudiment of the notochord. On either side a line indicates the cleavage of
the mesoblast just commencing. This cleavage, it will be seen, does not exist
in the more central parts of the embryo.
In C, which represents a vertical longitudinal section of later date, both
head-fold (on the right) and tail-fold (on the left) have advanced considerably.
The alimentary canal is therefore closed in both, in front and behind, but is in
middle still widely open to the yolk y below. Though the axial parts of the
embryo have become thickened by growth, the body-walls are still thin; in
them however is seen the cleavage of the mesoblast, and the divergence of
the somatopleure and splanchnopleure. The splanchnopleure both at the head
and at the tail is folded in to a greater extent than the somatopleure, and
PP-1-,::.
E
It
^
II.]
THE BODY-FOLDS. 31
forms the still wide splanchnic stalk. At the end of the stalk, which is as yet
short, it bends outwards again and spreads over the top of the yolk. The
somatopleure folded in less than the splanchnopleure to form the wider somatic
stalk, sooner bends round and runs outwards again. At a little distance from
both the head and the tail it is raised up into a fold, af, af, that in front of the
head being the highest. These are the amniotic folds. Descending from either
fold, it speedily joins the splanchnopleure again, and the two, once more united
into an un cleft membrane, extend some way downwards over the yolk, the
limit or outer margin of the opaque area not being shewn. All the space
between the somatopleure and the splanchnopleure is shaded with dots, pp.
Close to the body this space may be called the pleuroperitoneal cavity; but
outside the body it runs up into either amuiotic told, and also extends some
little way over the yolk.
D represents the tail-end at about the same stage on a more enlarged
scale, in order to illustrate the position of the allantois al (which was for the
sake of simplicity omitted in (7), shewn as a bud from the splanchnopleure,
stretching downwards into the pleuroperitoneal cavity pp. The dotted area
representing as before the whole space between the splanchnopleure and the
somatopleure, it is evident that a way is open for the aliautois to extend from
its present position into the space between the two walls of the amniotic
fold af.
E, also a longitudinal section, represents a stage still farther advanced.
Both splanchnic and somatic stalks are much narrowed, especially the former,
the cavity of the alimentary canal being now connected with the cavity of the
yolk by a mere canal. The folds of the amnion are spreading over the top of
the embryo and nearly meet. Each fold consists of two walls or Jimbs, the
space between which (dotted) is as before merely a part of the space between
the somatopleure and splanchnopleure. Between these arched amniotic folds
and the body of the embryo is a space not as yet entirely closed in.
F represents on a different scale a transverse section of E taken through
the middle of the splanchnic stalk. The black ring in the body of the embryo
shews the position of the neural canal, below which is a black spot, marking
the notochord. On either side of the notochord the divergence of somato-
pleure and splanchnopleure is obvious. The splanchnopleure, more or less
thickened, is somewhat bent in towards the middle line, but the two sides do
not unite, the alimentary canal being as yet open below at this spot; after
converging somewhat they diverge again and run outwards over the yolk.
The somatopleure folded in to some extent at first to form the body-walls
(which are here made too thick), soon bends outwards again, and almost im-
mediately is raised up into the lateral folds of tbe amnion af. The conti-
nuity of the pleuroperitoneal cavity within the body with the interior of the
amniotic fold outside the body is evident; both cavities are dotted. It will
of course be understood that this is a purely diagrammatic representation,
the various cavities, &c., being exaggerated in order to shew their relations more
clearly.
G, which corresponds to D at a later stage, is introduced to shew the
manner in which the allantois, now a distinctly hollow body, whose cavity is
continuous with that of the alimentary canal, becomes directed towards the
amniotic fold.
In H a longitudinal, and / a transverse section of later date, great changes
have taken place. The several folds of the amnion have met and coalesced
above the body of the embryo. The inner limbs of the several folds have
united into a single membrane (a), which encloses a space (ae) round the embryo.
This membrane a is the amnion proper, and the cavity within it, i.e. between
it and the embryo, is the cavity of the amnion containing the liquor amnii.
32
PRELIMINAKY ACCOUNT.
[CHAP.
It will be seen that the amnion a now forms in every direction the termina-
tion of the somatopleure ; the peripheral portions of the sornatopleure, the
united outer or descending limbs or walls of the folds af in (7, D, F, G having
been cut adrift, and now forming an independent continuous membrane, the
chorion, immediately underneath the vitelline membrane.
Jn / the splanchnopleure is seen converging to complete the closure
of the alimentary canal a', even at the stalk (elsewhere the canal has of course
long been closed in), and then spreading outwards as usual over the yolk.
The point at which it unites with the somatopleure, marking the extreme
limit of the cleavage of the mesoblast, is now much nearer the lower pole
of the diminished yolk.
As a result of these several changes, a great increase in the dotted space
has taken place. It is now possible to pass from the actual peritoneal cavity
within the body, on the one hand round a great portion of the circumference
of the yolk, and on the other hand above the amnion a, in the space between
it and the chorion.
Into this space the allantois is seen spreading in K at al.
M
In L the splanchnopleure has completely invested the yolk-sac, but at the
lower pole of the yolk is still continuous with that peripheral remnant of the
soraatopleure now called the chorion. In- other words, cleavage of the meso-
blast has been carried all round the yolk (ys) except at the very lower pole.
In M the cleavage has been carried through the pole itself; the peripheral
portion of the splanchnopleure forms a complete investment of the yolk quite
unconnected with the peripheral portion of the somatopleure, which no*w exists
as a continuous membrane lining the interior of the shell. The yolk sac (ys) is
therefore quite loose in the pleuroperitoneal cavity, being connected only with
the alimentary canal (a') by a solid pedicle.
II.]
THE HEAD-FOLD.
33
Lastly, in N the yolk sac (?/s) is shewn being withdrawn into the cavity
of the body of the embryo. The allantois is as before, for the sake of
simplicity, omitted ; its pedicle would of course lie by the side of ys in the
somatic stalk marked by the usual dotted shading.
It may be repeated that the above are diagrams, the various spaces being
shewn distended, whereas in many of them in the actual egg the walls have
collapsed, and are in near juxtaposition.
c
FIG. 9.
JVC.
F.So.
DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF AN EMBRYO.
The section is supposed to be made at a time when the head-fold has com-
menced but the tail-fold has not yet appeared.
F. So. fold of the somatopleure. F. Sp. fold of the splanchnopleure.*
The line of reference F. So. is placed in the lower bay, outside the embryo.
The line of D is placed in the upper bay inside the embryo; this will remain as
the alimentary canal. Both folds (F. So., F. Sp.) are parts of the head-fold, and
are to be thought of as continually travelling onwards (to the left) as develop-
ment proceeds.
pp. space between somatopleure and splanchnopleure: pleuroperitoneal cavity.
Am. commencing (head) fold of the amnion.
A fuller explanation is given under Fig. 16.
forward, with a gentle ascent, regains the original level. As
seen in section, then, the blastoderm at this spot may be said to
be folded up in the form of the letter 8. This fold we shall
always speak of as the head-fold. In it we may recognize
two limbs : an upper limb in which the curve is directed for-
wards, and its bay, opening backwards, is underneath the
blastoderm, i. e. as we shall see, inside the embryo (Fig. 9, D]
and an under limb in which the curve is directed backwards,
and its bay, opening forwards, is above the blastoderm, i.e.
outside the embryo. If an j like the above, made of some
elastic material, were stretched laterally, the effect would be
to make both limbs longer and proportionally narrower, and
E. 3
34 PRELIMINARY ACCOUNT. [CHAP.
their bays, instead of being shallow cups, would become more
tubular. Such a result is in part arrived at by the growth
of the blastoderm; the upper limb of the 8 is continually
growing forward (but, unlike the stretched elastic model, in-
creases in all its dimensions at the -same time), and the lower
limb is as continually lengthening backwards ; and thus both
upper and lower bays become longer and longer. This we
shall hereafter speak of as the travelling backwards of the
head-fold.
The two bays do not however both become tubular. The
section we have been speaking of is supposed to be taken
vertically along a line, which will afterwards become the axis
of the embryo ; and the lower bay of the 3 is a section of the
crescentic groove mentioned above, in its middle or deepest
part. On either side of the middle line the groove gradually
becomes shallower. Hence in sections taken on either side
of the middle line or axis of the embryo (above or below the
plane of the figures), the groove would appear the less marked
the farther the section from the middle line, and at a certain
distance would disappear altogether. It must be remembered
that the groove is at first crescent-shaped, with its concavity
turned towards what will be the hind end of the embryo
(Fig. 11). As the whole head-fold is carried farther and farther
back, the horns of the crescent are more and more drawn
in towards the middle line, the groove becoming first semi^
circular, then horse-shoe-shaped. In other words, the head-
fold, instead of being a simple fold running straight back-
wards, becomes a curved fold with a central portion in front
running backwards, and two side portions running in towards
the middle line. The effect of this is that the upper bay of
the 8 (that within the embryo) gets closed in at the sides
as well as in the front, and thus speedily becomes tubular.
The under bay of the 8 (that outside the embryo) remains
of course open at the sides as in front, and forms a sort of
horse-shoe-shaped ditch surrounding the front end of the
embryo.
We have dwelt thus at length on the formation of the
head-fold, because, unless its characters are fairly grasped,
much difficulty may be found in understanding many events
in the history of the chick. The reader will perhaps find the
matter easier to comprehend if he makes for himself a rough
II.] THE EMBRYONIC SAC. 35
model, which he easily can do by spreading a cloth out flat
to represent the blastoderm, placing one hand underneath it,
to mark the axis of the embryo, and then tucking in the
cloth from above under the tips of his fingers. The fingers,
covered with the cloth and slightly projecting from the level
of the rest of the cloth, will represent the head, in front of
which will be the semicircular or horse-shoe-shaped groove
of the head-fold.
At its first appearance the whole 8 may be spoken of as
the head-fold, but later on it will be found convenient to
restrict the name chiefly to the lower limb of the 8.
Some time after the appearance of the head-fold, an
altogether similar but less conspicuous fold makes its ap-
pearance, at a point which will become the posterior end of
the embryo. This fold, which travels forwards just as the
head-fold travels backwards, is the tail-fold (Fig. 8, C).
In addition, between the head- and the tail-fold two lateral
folds appear, one on either side. These are simpler in cha-
racter than either head-fold or tail-fold, inasmuch as they
are nearly straight folds directed inwards towards the axis of
the body (Fig. 8, F), and not complicated by being crescentic
in form. Otherwise they are exactly similar.
As these several folds become more and more developed,
the head-fold travelling backwards, the tail-fold forwards,
and the lateral folds inwards, they tend to unite in the
middle point ; and thus give rise more and more distinctly
to the appearance of a small tubular sac seated upon, and
connected, by a continually-narrowing hollow stalk, with that
larger sac which is formed by the extension of the rest of the
blastoderm over the whole yolk.
The smaller sac we may call the "embryonic sac," the
larger one "the yolk-sac." As incubation proceeds the smaller
sac (Fig. 8), gets larger and larger at the expense of the yolk-
sac (the contents of the latter being gradually assimilated by
nutritive processes into the tissues forming the growing walls
of the former, not directly transferred from one cavity into
the other). Within a day or two of the hatching of the
chick, at a time when the yolk-sac is still of some consider-
able size, or at least has not yet dwindled away altogether,
and the development of the embryonic sac is nearly com-
plete, the yolk-sac (Fig. 8, JV) is slipped into the body of
32
36 PRELIMINARY ACCOUNT. [CHAP-
the embryo, so that ultimately the embryonic sac alone re-
mains.
6. The embryo, then, is formed by a folding off of a
portion of the blastoderm from the yolk-sac. The general
outline of the embryo is due to the direction and shape of
the several folds which share in its formation ; these, while
preserving a nearly perfect bilateral symmetry, present marked
differences at the two ends of the embryo. Hence from the
very first there is no difficulty in distinguishing the end which
will be the head from that which will be the tail.
In addition to this, the tubular sac of the embryo, while
everywhere gradually acquiring thicker and thicker walls,
undergoes at various points, through local activities of growth
in the form of thickenings, ridges, buds or other processes,
many modifications of the outline conferred upon it by the
constituent folds. Thus bud- like processes start out from
the trunk to form the rudiments of the limbs, and simila,r
thickenings and ridges give rise to the jaws and other parts
of the face. By the unequal development of these outgrowths
the body of the chick is gradually moulded into its proper
outward shape.
7. Were the changes which take place of this class only,
the result would be a tubular sac of somewhat complicated
outline, but still a simple tubular sac. Such a simple sac
might perhaps be roughly taken to represent the body of
many an invertebrate animal ; but the typical structure of a
bird or other vertebrate animal is widely different. It may
very briefly be described as follows.
First there is, above, a canal running lengthways along
the body, in which are lodged the brain and spinal cord.
Below this neural tube is an axis represented by the bodies
of the vertebra and their continuation forwards in the
structures which form the base of the skull. Underneath
this, again, is another tube closed in above by the axis,
and on the sides and below by the body- walls. Enclosed
in this second tube, and suspended from the axis, is a
third tube, consisting of the alimentary canal with its
appendages (liver, salivary glands, lungs, &c., which are
fundamentally mere diverticula from one simple canal).
The cavity of the outer tube, which also contains the
heart and other parts of the vascular system, is the general
II.] , THE NEURAL TUBE. 37
body cavity ; it is divided into a thoracic or pleural, and an
abdominal or peritoneal cavity; these two cavities are, how-
ever., from their mode of origin, portions of one and the same
tube. Thus a transverse section of a vertebrate animal
always shpws the same fundamental structure: above a single
tube, below a double tube, the latter consisting of one tube
enclosed within another, the inner being the alimentary
canal, the outer the general cavity of the body. Into such
a triple tube the simple tubular embryonic sac of the chick
is converted by a series of changes of a remarkable character.
The upper or neural tube is formed in the following way.
At a very early period the upper surface of the blastoderm
in the region which will become the embryo, is raised up
into two ridges or folds which run parallel to each other at a
short distance on either side of what will be the long axis of
the embryo, and thus leave between them a shallow longitu-
dinal groove (Fig. 8, B, also Figs. 11, 12, m.c). As these ridges,
which bear the name of medullary folds, increase in height
they arch over towards each other and eventually meet
and coalesce in the middle line, thus converting the groove
into a canal, which at the same time becomes closed at either
end (Fig. 8, F, /, also Fig. 13. .M.). The cavity so
formed is the cavity of the neural tube, and eventually
becomes the cerebro-spinal canal.
The lower double tube, that of the alimentary canal, and
of the general cavity of the body, is formed in an entirely
different way. It is, broadly speaking, the result of the junc-
tion and coalescence of the fundamental embryonic folds, the
head-fold, tail-fold, and lateral folds ; in a certain sense the
cavity of the body is the cavity of the tubular sac described
in the last paragraph.
But it is obvious that a tubular sac formed by the folding
in of a single sheet of tissue, such as we have hitherto con-
sidered the blastoderm to be, must be a simple tubular sac
possessing a single cavity only. The blastoderm however
does not long remain a single sheet, but speedily becomes a
double sheet of such a kind that, when folded in, it gives rise
to a double tube.
Very early the blastoderm becomes thickened in the
region of the embryo, the thickening being chiefly due to an
increase in the middle layer or mesoblast, while at the same
SS PRELIMINARY ACCOUNT. [CHAP.
time it becomes split or cleft horizontally over the greater
part of its extent into two leaves, an upper leaf and a lower
leaf. In the neighbourhood of the axis of the body, beneath
the neural tube, this cleavage is absent (Fig. 8, B\ also Figs.
13 20), in fact, it begins at some little distance on either
side of the axis and spreads thence into the periphery in all
directions. It is along the thickened mesoblast that the
cleavage takes place, the upper part of the mesoblast uniting
with epiblast to form the upper leaf, and the lower part with
the hypoblast to form the lower leaf.
In the fundamental folds both leaves are involved, both
leaves are folded downwards and inwards, both leaves tend
to meet in the middle below; but the lower leaf is folded in
more rapidly, and thus diverges from the upper leaf, a space
being gradually developed between them (Fig. 8). In course
of time the several folds of the lower leaf meet and unite to
form an inner tube quite independently of the upper leaf>
whose own folds in turn meet and unite to form an outer
tube separated from the inner one by an intervening space.
The inner tube is the alimentary canal which is subsequently
perforated at both ends to form the mouth and anus; the
walls of the outer tube are the walls of the body, and the
space between the two tubes is the general " serous cavity/'
which being subsequently divided into pleural and peritoneal
portions, may be spoken of as the pleuroperitoneal cavity.
Hence the upper (or outer) leaf of the blastoderm, from
its giving rise to the body- walls, is called the somatopleure 1 ;
the lower (or inner) leaf, from its forming the alimentary
canal and its tributary viscera, the splanchnopleure 2 .
This horizontal splitting of the blastoderm into a somato-
pleure and a splanchnopleure, which we shall hereafter speak
of as the cleavage of the mesoblast, is not confined to the region
of the embryo, but gradually extends over the whole of the
yolk-sac. Hence in the later days of incubation the yolk-
sac comes to have two distinct coats, an inner splanclmo-
pleuric and an outer somatopleuric investment, separable
from each other all over the sac. We have seen that, owing
to the manner of its formation, the ' embryonic sac ' is con-
nected with the ' yolk-sac' by a continual narrowing hollow
1 Soma, body, pleuron, side. ? Splanchnic, viscus, pleuron y side.
II.] THE AMNION. 39
stalk ; but this stalk must, like the embryonic sac itself, be
a double stalk, and consist of a smaller inner stalk within a
larger outer one, Fig. 8, E } H. The folds of the splanchnopleure,
as they tend to meet and unite in the middle line below,
give rise to a continually narrowing hollow stalk of their
own, a splanchnic stalk, by means of which the walls of the
alimentary canal are continuous with the splanchnopleuric
investment of the yolk-sac, and the interior of that canal is
continuous with the yolk inside the yolk-sac. In the same
way the folds of the somatopleure form a similar stalk of
their own, a somatic stalk, by means of which the body-
walls of the chick are continuous (for some time; the con-
tinuity, as we shall see, being eventually broken by the
development of the amnion) with the somatopleuric invest-
ment of the yolk-sac ; and the pleuroperitoneal cavity of the
body of the chick is continuous with the narrow space be-
tween the two investments of the yolk-sac.
At a comparatively early period the canal of the splanch-
nic stalk becomes obliterated, so that the material of the
yolk can no longer pass directly into the alimentary cavity,
but has to find its way into the body of the chick by absorp-
tion through the blood-vessels. The somatic stalk, on the
other hand, remains widely open for a much longer time ; but
the somatic shell of the yolk-sac never undergoes that thick-
ening which takes place in the somatic walls of the embryo
itself; on the contrary, it remains thin and insignificant.
When accordingly in the last days of incubation the greatly
diminished yolk-sac with its splanchnic investment is with-
drawn into the rapidly enlarging abdominal cavity of the
embryo, the walls of the abdomen close in and unite, without
any regard to the shrivelled, emptied somatopleuric invest-
ment of the yolk-sac, which is cast off as no longer of any
use. (Fig. 8. Compare the series.)
8. Very closely connected with the cleavage of the meso-
blast and the division into somatopleure and splanchnopleure,
is the formation of the amnion, all mention of which was, for
the sake of simplicity, purposely omitted in the description
just given.
The amnion takes its origin from certain folds of the
.somatopleure, and of the somatopleure only, in the following
way.
40 PRELIMINARY ACCOUNT. [CHAP.
At a time when the cleavage of the mesoblast has some-
what advanced, there appears, a little way in front of the semi-
lunar head-fold, a second fold (Fig. 11, also Fig 8, (7.), running
more or less parallel or rather concentric with the first and
not unlike it in general appearance, though differing widely
from it in nature. In the head-fold the whole thickness of the
blastoderm is involved ; in it both somatopleure and splanch-
nopleure (where they exist, i.e. where the mesoblast is cleft,)
take part. This second fold, on the contrary, is limited entirely
to the somatopleure. Compare Figs. 8 and 9. In front of
the head-fold, and therefore altogether in front of the body
of the embryo, the somatopleure is a very thin membrane,
consisting only of epiblast and a very thin layer of mesoblast ;
and the fold we are speaking of is, in consequence, itself
thin and delicate. Rising up as a semilunar fold with its
concavity directed towards the embryo (Fig. 8, (7, a/.),
as it increases in height it is gradually drawn backwards
over the developing head of the embryo. The fold thus
covering the head is in due time accompanied by similar
folds of the somatopleure, starting at some little distance be-
hind the tail, and at some little distance from the sides (Fig. 8,
C, D, E, F). In this way the embryo becomes surrounded
by a series of folds of thin somatopleure, which form a
continuous wall all round it. All are drawn gradually over the
body of the embryo, and at last meet and completely coalesce
(Fig. 8, H, /), all traces of their junction being removed.
Beneath these united folds there is therefore a cavity, within
which the embryo lies (Fig. 8, H y ae). This cavity is the
cavity of the amnion. The folds which we have been
describing are those which form the amnion.
Each fold, of course, necessarily consists of two limbs, both
limbs consisting of epiblast and a very thin layer of mesoblast;
but in one limb the epiblast looks towards the embryo, while
in the other it looks away from it. The space between the two
limbs of the fold, as can easily be seen in Fig. 8, is really part
of the space between the somatopleure and splanchnopleure ;
it is therefore continuous with the general space, part of which
afterwards becomes the pleuroperitoneal cavity of the body,
shaded with dots in the figure and marked (p p). So that it
is possible to pass from the cavity between the two limbs of
each fold of the amnion into the cavity which surrounds
II.] THE ALLANTOIS. i 41
the alimentary canal. When the several folds meet and
coalesce together above the embryo, they unite in such a
way that all their inner limbs go to form a continuous inner
membrane or sac, and all their outer limbs a similarly con-
tinuous outer membrane or sac. The inner membrane thus
built up forms a completely closed sac round the body of the
embryo, and is called the amniotic sac, or amnion proper,
(Fig. 8, H, /, &c. a.), and the fluid which it afterwards con-
tains is called the amniotic fluid, or liquor amnii. The space
between the inner and outer sac, being formed by the united
cavities of the several folds, is, from the mode of its forma-
tion, simply a part of the general cavity found everywhere
between somatopleure and splanchnopleure. The outer sac
over the embryo lies close under the vitelline membrane,
while its periphery is gradually extended over the yolk as
the somatopleuric investment of the yolk-sac described in the
preceding paragraph.
9. If the mode of origin of these two sacs (the inner
or true amnion, and the outer or false amnion, as Baer
called it) and their relations to the embryo be borne in mind,
the reader will have no difficulty in understanding the course
taken in its growth by an important organ, the allantois, of
which we shall hereafter have to speak more in detail. ^
The allantois is fundamentally an appendage of the
alimentary canal, and may be regarded as a bud thrown
out by the splanchnopleure close to its junction with the
somatopleure at the hinder end of the embryo (Fig. 8, D,
al.}. From thence it grows first into the pleuroperitoneal
cavity of the embryo, and thence very rapidly pushes its
way by the development of a long stalk into the space
between the true and false amniotic sacs (Fig. 8, (?, K}.
Curving over the embryo, it comes to lie over the embryo
and the amnion proper, separated from the shell (and vitelline
membrane) by nothing more than the thin false amnion.
In this position it performs its functions as a respiratory
organ. It is evident that though now placed quite outside
the embryo, the space in which it lies is a continuation of
that peritoneal cavity in which it took its origin.
It is only necessary to add, that the false amnion either
coalesces with the vitelline membrane, in contact with which
42 PRELIMINARY ACCOUNT. [CHAP. II.
it lies, or else replaces it, and in the later days of incuba-
tion is known as the chorion.
In the above account we Lave described the somatopleure as consisting
of mesoblast as well as epiblast even in its most peripheral portions. The inner
limbs of the amrriotic folds undoubtedly contain mesoblastic elements, since
the amnion proper contains plain muscular fibres. Some authors however
regard the outer limbs of the amniotic folds (giving rise to the false amnion)
and the somatopleuru beyond them as being composed of epiblast only.
CHAPTER III.
THE CHANGES WHICH TAKE PLACE DURING THE FIRST
DAY OF INCUBATION.
1. DURING the descent of tlie egg along the oviduct,
where it is exposed to a temperature of about 40 C, the
blastoderm, as we have seen, continues to undergo im-
portant changes. When the egg is laid and becomes cold
these changes all but entirely cease, and the blastoderm
remains inactive until, under the influence of the higher tem-
perature of natural or artificial incubation, the vital activities
of the germ are brought back into play, the arrested changes
go on again, and usher in the series of events which we have
now to describe in detail.
The condition of the blastoderm at the time when the
egg is laid is not exactly the same in all eggs, in some the
changes being farther advanced than in others, though the
differences of course are slight ; in some eggs, especially in
warm weather, changes of the same kind as those caused by
actual incubation may take place, to a certain extent, in the
interval between laying and incubation ; lastly, in all eggs,
both under natural and especially under artificial incubation,
the dates of the several changes are, within the limits of
some hours, very uncertain, particularly in the first few days ;
one egg being found, for example, at 36 hours in the same
stage as another at 24 or 30 hours, or a third at 40 or 48
hours. When we speak therefore of any event as taking
place at any given hour or part of any given day, we are to
be understood as meaning that such an event will generally
be found to have taken place at about that time. We
introduce exact dates for the convenience of description.
44 THE FIRST DAY. [CHAP.
The changes which take place during the first day will be
most easily considered under three periods : from the 1st to
the 12th, from the 12th to the 20th, and from the 20th to the
24th hour.
2. From the 1st to about the 1.2th hour. During this
period the blastoderm when viewed from above is found to
have increased greatly in size. The pellucid area, which at
the best is but obscurely marked in the unincubated egg,
becomes very distinct (the central opacity having dis-
appeared), and contrasts strongly with the opaque area,
which has even still more increased both in distinctness
and size.
For the first few hours both the pellucid and opaque
areas remain circular, and the only change, besides increase
in size and greater distinctness which can be observed in
them, is a slight ill-defined opacity or loss of transparency,
which makes its appearance in about the middle of the
pellucid area. This is known as the embryonic shield.
3. Slight as are the changes which can at this stage be
seen from surface views, sections taken from hardened
specimens bring to light many most important changes in
the nature and arrangement of the constituent cells.
It will be remembered that the blastoderm in the un-
incubated egg is composed of two layers, an upper (Fig. 3, ep.)
and an under layer; that the upper is a coherent membrane
of columnar nucleated cells, but that the lower one (Fig. 3, 1.)
is formed of an irregular network of larger cells in which
the nuclei, if present, are rarely visible ; and that in addition
to this there are certain still larger cells, called ' formative
cells ' (Fig. 3, 6), lying at the bottom of the segmentation-
cavity.
Under the influence of incubation changes take place
very rapidly, which result in the formation of the three
layers of the blastoderm.
The upper layer, which we shall henceforward call the
epiblast (Fig. 10, A), takes but little share in these changes.
In the lower layer, however, certain of the cells begin
to get flattened horizontally, their granules become less
numerous, and a distinct nucleus makes its appearance in
them; the cells so altered cohere together and form a mem-
brane (Fig. 10, (7). The membrane thus formed, which is
III.] THE MESOBLAST. 45
first completed in the centre of the pellucid area, we shall
henceforward speak of as the hypoblast.
Between it and the epiblast many of the cells of the
original lower layer are enclosed, and in addition some of the
formative cells (migrating by help of amoeboid movements
after the fashion of white-blood corpuscles) begin to travel
round the edge of the hypoblast, and to pass in between it
and the epiblast.
The cells, whether originally "formative" cells or cells
from the lower layer, thus gathered between the epiblast and
hypoblast, undergo a process of endogenous cell-formation, by
which the whole of the interior of each becomes converted
into a number of new cells, These new cells, spherical in
form, and possessing a large nucleus with a distinct nucleolus,
are first formed in the centre of the pellucid area and sub-
sequently in its periphery. They constitute the third layer
or mesoUast (Fig. 10, E).
The epiblast is the Horriblatt (corneal layer), and the hypoblast the Darm-
drusenblatt (epithelial glandular layer) of the Germans, while those parts of the
mesoblast which take part in the formation of the somatopleure and splanchno-
pleure correspond respectively to the Haut-muskd-platte and Darm-faser-platle.
All blood-vessels arise in the mesoblast. Hence the vascular layer of the
older writers fall entirely within the mesoblast.
The serous layer of the same authors includes the whole of the epiblast, but
also comprises a certain portion of mesoblast; for they speak of all the organs of
animal life (skin, bones, muscle, &c.) as being formed out of the serous layer,
whereas the epiblast proper gives rise only to the epidermis and to certain
parts of the nervous system. In the same way their mucous layer corresponds
to the hypoblast with so much of the mesoblast as takes part in the formation of
the organs of organic life. Their vascular layer therefore answers to a part only
of the mesoblast, viz. that part in which blood-vessels are especially developed.
It is worthy of notice that the cells of the epiblast are themselves the
direct results of segmentation ; but that the hypoblast and mesoblast are
formed at a subsequent period, and are therefore only indirectly the results of
segmentation. The true difference between the hypoblast and mesoblast lies
in the mode in which each layer is formed, and not in any essential differ-
ence in the segmentation-spheres from which each is derived.
At about the time when the hypoblast is completely
formed as a distinct membrane, the mesoblast cells form a
somewhat thick mass in the centre of the blastoderm, and
cause the central opacity spoken of above as the embryonic
shield.
4. Soon after this, between the 8th and 12th hours, the
hitherto circular pellucid area becomes oval (the opaque
area remaining circular) . The oval is, with remarkable regu-
4G
THE FIRST DAY.
[CHAP.
larity, so placed that its long axis forms a right angle, or
very nearly a right angle, with the long axis of the egg
itself. Its narrow end corresponds with the future hind end
of the embryo : and we may henceforward speak of it as the
hind end. If an egg be placed with its broad end to the
right hand of the observer, the head of the embryo will in
nearly all cases be found pointing away from him.
At about the time when the pellucid area is beginning
to undergo this change of shape, there appears in surface
views, along a line corresponding with the long axis of
the oval, and occupying not, as might perhaps be expected,
its front but its hinder two-thirds, a narrow opaque streak,
much more opaque, and therefore distinct, than the em-
bryonic shield, but still shadowy and ill-defined. This is
known as the primitive streak.
FIG. 10.
SECTION OF A BLASTODERM AT RIGHT ANGLES TO THE LONG AXIS OF TEE
EMBRYO AFTER EIGHT HOURS' INCUBATION.
(Taken about midway between front and hind end.)
A. epiblast. B. mesoblast. C. hypoblast. pr. primitive groove. /. fold in
the blastoderm, probably produced by the action of the chromic acid.
m. c. mesoblast cell; the line points to one of the peripheral mesoblast
cells lying between epiblast and hypoblast. Id. formative cells.
The following are the chief points represented in the section, (i) The
thickening of the mesoblast underneath the primitive groove pr., even when
it is hardly at all present at the sides of the groove. (2) The hypoblast, C, early
formed as a single layer of spindle-shaped cells. (3) The so-called segmentation -
cavity, in which coagulated albumen is present. On the floor of this are the
large formative cells bd.
The line of separation between the epiblast and mesoblast underneath the
primitive groove is too strongly marked in the figure.
The primitive streak is no sooner formed than it becomes
marked on its upper surface by a delicate shallow furrow
III.] THE PRIMITIVE GROOVE. 47
running along its axis. In fresh specimens viewed with
transmitted light, this furrow appears as a linear trans-
parency, but in hardened specimens seen under reflected
light may be distinctly recognized as a furrow or narrow
groove, the bottom of which being thinner than the sides
appears more transparent when viewed with transmitted
light. It is known as the primitive groove. The nature of
the changes by which it is brought about can only be learnt
by the study of vertical sections (Fig. 10). These teach us
that the opacity which marks out the primitive streak is
chiefly due to a thickening of the mesoblast. In the for-
mation, however, both of the primitive streak, and especially
of the primitive groove, the epiblast also plays an important
part.
During these twelve hours the epiblast has been spreading
rapidly, much more rapidly than the other two layers. Over
the white yolk in the region of the opaque area it forms a
layer one cell deep, but at the same time has become two or
three cells deep in the centre of the pellucid area. In the
pellucid area its constituent cells have become narrower (6/^)
and more columnar, but over the opaque area flatter and
broader (12 //,) than they were at first. At the 12th hour
therefore we find a distinct histological difference between
the epiblast cells of the pellucid and those of the opaque
area.
Over the thickening of the mesoblast, which forms the
basis of the primitive streak, the epiblast is also thickened ;
the hypoblast, however, remains here, as in the rest of the
blastoderm, a flat sheet consisting of a single layer of flat-
tened (seen in sections as a single row (Fig. 10, G) of spindle-
shaped) cells, which become larger ^nd more irregular at the
periphery. The thickening of the mesoblast and epiblast
in the region of the primitive streak causes the upper
outline of the blastoderm as seen in sections to rise above
the general surface in a gentle curve (Fig. 10).
The primitive groove is formed almost entirely by a
pushing in or depression of the epiblast at the summit of
this curve.
The thickness of the epiblast remains about the same on the sides as at the
bottom of the groove. The mesoblast, on the contrary, is thinner immediately
beneath the bottom of the groove than at the two sides, where it is decidedly
48 THE FIKST DAY. [CHAP.
thicker than in the rest of the pellucid area. It is apparently this median
thinning of the mesoblast which gives rise to the linear transparency seen in
specimens viewed with transmitted light. The hypoblast, it may be remarked,
is generally curved downwards beneath the primitive streak and groove, though
not to the same extent as the epiblast. Thus the whole blastoderm is some-
what curved in this region. Immediately beneath the groove a kind of fusion
takes place between the epiblast and mesoblast, though on close examination
the line of junction between them can generally be made out. "This apparent
fusion His (Ueber die Erste Anlage des Wirbeltheirleibs) regarded as aw event
of great importance, and gave the name of axis-cord to the part in which it
occurs. In fresh specimens a narrow (opaque) streak can be seen running
down the centre of the groove; but it is not represented by any structure
which can be seen in sections.
The chief events then which occur during the first twelve
hours of incubation are the establishment of the three layers
of the blastoderm, and the appearance of the embryonic
shield, of the primitive streak and of the primitive groove.
5. From the 12th to the 20th hour. During this period
the pellucid area rapidly increases in size, and from being
oval becomes pear-shaped. The primitive groove grows even
more rapidly than the pellucid area; so that by the 16th hour
it is not only absolutely, but also relatively to, the pellucid
area, longer than it was at the 12th hour. . ; The interval
between its end and the circumference of the pellucid area
continues to be greater in front than behind.
At about the 16th hour, or a little later, a thickening of
the mesoblast takes place in front of the primitive groove,
giving rise to an opaque streak ending abruptly in front
against a semicircular fold, which appears at this time near
the anterior extremity of the pellucid area (Fig. 11), and
is known as the head-fold. In fresh specimens this streak
looks like a continuation from the anterior extremity of the
primitive groove ; but in hardened specimens it is easy to
see that the connection is only an apparent one.
Along the new streak a groove (Fig. 11, m. c.) is very soon
formed, which, narrow in front, but widening very much
behind, embraces between its diverging walls the anterior
extremity of the primitive groove. This new groove, by the
conversion of which into a tube the medullary canal will be
formed, is known as the medullary groove.
On each side of it the mesoblast is thickened, and the
surface of the blastoderm raised up in the form of two longi-
tudinal folds, known as the lamince dorsales, or the medullary
folds (Fig. 11, -4). Immediately beneath the bottom of the
III.]
THE NOTOCHORD.
Fia. ii.
SURFACE VIEW OF THE PELLUCID AREA OF A BLASTODERM OF 18 HOURS.
None of the opaque area is shewn, the pear-shaped outline indicating the
limits of the pellucid area.
At the hinder part of the area is seen the primitive groove pr., with its
nearly parallel walls, fading away behind, but curving round and meeting in
front so as to form a distinct anterior termination to the groove, about half way
up the pellucid area.
Above the primitive groove is seen the medullary groove m. c., with the
medullary folds A. These diverging behind, slope away on either side of the
primitive groove, while in front they curve round and meet each other close
upon a curved line which represents the head-fold.
The second curved line in front of and concentric with the first is the com-
mencing fold of the amnion.
groove, however, the mesoblast is thinned out and very soon
the cells in this position, separating from the lateral masses,
adhere together in the middle line, and thus form between
the epiblast and the hypoblast a flattened circular rod known
as the notochord, seen in section as an elliptical aggregation
of cells (Fig. 12, cA.)
E. 4
50 *THE FIRST DAY. [CHAP.
The medullary groove differs in many important particulars from the primi-
tive groove. Beneath the primitive groove the mesoblast always fuses more
or less with the epiblast ; this is never the case under the medullary groove.
Under the primitive groove the mesoblast never shews any signs of differ-
entiation into any organ ; under the medullary groove the notochord is formed
out of the mesoblast cells. The epiblast lining the bottom of the medullary
groove frequently becomes very much thinner than at its sides ; this seems
never to be the case with the primitive groove.
The primitive groove reaches its maximum growth before the appearance
of the medullary groove ; and after the appearance of the latter gradually
becomes less and less conspicuous, and finally disappears without leaving a
trace. A curved remnant of it is to be tound at the hind end of the medul-
lary canal between the 3oth and 4Oth hours, but by the 5Oth not a trace of it
remains.
By the earlier observers the primitive groove was supposed to become con-
verted into the medullary canal. Dursy (Der Primitivstreif des Hiihnchens)
was the first to give a correct account of its disappearance; and the distinction
between it and the medullary groove has since been fully recognized by many
observers. Goette (Archiv. Micr. Anat. Vol. x. 1873, PP- J 45 f 99) describes
the medullary groove as always appearing to the left of the primitive groove,
and having its floor continuous with the left wall of the latter. He states that
beneath this left wall the unsymmetrically placed axis-cord is found ; indeed
he considers that the notochord is a forward continuation of the axis-cord, and
that the latter, as the primitive groove recedes before the medullary groove,
becomes continuously converted into the former.
The primitive groove then is a structure which appears early, and soon
disappears without entering directly into the formation of any part of the
future animal. Apparently it has no function whatever. We can only sup-
pose that it is the rudiment of some ancestral feature.
6. By the 20th hour the medullary groove or canal ; with
its medullary folds or laminae dorsales, is fully established. It
then presents the appearance, towards the hinder extremity
of the embryo, of a shallow groove with sloping diverging
walls which embrace between them the remains of the
vanishing primitive groove.
Passing forwards towards what will become the head of
the embryo the groove becomes narrower arid deeper with
steeper walls. On reaching the head-fold (Fig. 11), which
continually becomes more and more prominent, the medul-
lary folds curve round and meet each other in the middle
line, so as to form a somewhat rounded end to the groove.
In front therefore the canal does not become lost by the
gradual flattening and divergence of its walls as is the case
behind, but has a definite termination, the limit being
marked by the head-fold.
In front of the head-fold, quite out of the region of
the medullary folds, there is usually another small fold
which is the beginning of the amnion (Fig. 11),
III.] THE HYPOBLAST. 51
We must now go back, and say a few words about the
changes which the cells of the various layers undergo from
the 12 20 hours.
FIG. 12.
f/i,
TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS.
The section passes through the medullary groove me., at some distance
behind its extreme front, and shews some of the chief points in which it differs
from the primitive groove.
The chief of these are, (i) the presence underrieath it of the notoohord ch. 9
(2) the absence of any apparent adhesion between the epiblast and the meso-
blast, (3) the thickening of the mesoblast underrieath the medullary folds, mf.
A. epiblast. B. mesoblast. C. hypoblast.
m. c. medullary groove. nt< /. medullary fold. ch. notochord : the small group
of mesoblast cells separated by a narrow gap from the thicker mass of
mesoblast on either side.
It is to be noticed that the cells of the hypoblast become more columnar as
they approach the edge of the pellucid area, and finally pass, without any strong
line of demarcation, into the white-yolk spheres.
Only one half of the section is represented if completed the section would
be symmetrical about the line passing through the centre of the medullary
canal, me. ,
The hypoblast (Fig. 12, C) continues to be only one cell
deep ; the cells being, during the whole of this period, flatter
in the centre, and larger and more irregular towards the peri-
phery of the blastoderm. At about the 12th hour they are
very irregular in size ; shewing very great variations over
a very small space. This probably implies that they are
rapidly undergoing division. Later, however (about the
18th hour), they are fairly uniform over particular regions,
though they vary considerably in size at different parts of
the pellucid area. In no case does the hypoblast extend
beyond the edge of the pellucid area.
The hypoblast cells along the central axis of the pellucid area, and for some
little distance on each side, are smaller than elsewhere over the blastoderm.
Over a small district just outside the embryo, and at about one-third of the
way from the posterior extremity of the blastoderm, they are, from the i8th 231x1
hour, considerably larger than anywhere else. The remaining hypobiast cells
42
52 THE FIRST DAY. [CHAP.
are intermediate in size between these very large cells and the smaller cells
in the centre. During the whole of this period the hypoblast cells continue
to be granular and filled with highly refractive spherules, exhibiting in this
respect a marked contrast to their appearance at a later time.
Their mode of increase is partly by division, but the layer grows chiefly
in a manner which is very different and somewhat remarkable. Before the
1 2th hour the hypoblast at its margin ended abruptly against the white-yolk
cells; but after that hour its relation to the white yolk becomes altered. As
they approach the white yolk the cells of the hypoblast become more and
more filled with white-yolk spherules, and at the extreme edge of the pellucid
area it is very difficult to say where the white yolk ends, and where the
hypoblast begins. This is somewhat diagrammatically shewn in Fig. 12. The
white-yolk spheres near the edge of the pellucid area have generally acquired
nuclei, though it is frequently difficult to see them owing to the numerous
highly refractive spherules which the spheres contain. The nearer they are
to the edge of the pellucid area the fewer spherules they contain, and at the
very edge it is almost impossible to say whether they ought to be called white-
yolk spheres or hypoblaat cells. The chief increase of the hypoblast therefore
seems to take place through the conversion, cell for cell, of the white yolk
into the hypoblast.
During this period the mesoblast (Fig. 12, J?) cells do
not undergo any marked change. The layer itself enlarges
to a certain extent through the multiplication of cells by
the division of old ones; but the chief increase in bulk is
probably due to the formative cells, which are continually
passing round from the bottom of the segmentation-cavity
to the mesoblast, and there become converted, in the way
described ( 3) above, into mesoblast cells.
These formative cells are more numerous at the bottom of the segmentation-
cavity at the 18th hour than they were at the first hour. This accession to
their number is probably due to fresh ones being formed from the floor of white
yolk. They appear to grow in size by absorbing the white-yolk spherules,
with which indeed they are completely filled.
The epiblast cells (Fig. 12, A) probably increase entirely
by division, and seem to derive their nourishment from the
white yolk on which the peripheral cells rest, and perhaps
also from the albuminous fluid which fills the segmentation-
cavity and occupies all the interstices between the cells of
the various layers.
The cells near the edge of the opaque area are the largest and flattest
of the epiblast cells; those in the middle of the pellucid area are smaller than
those at its edge.
Outside the blastoderm there are to be seen on the surface of the yolk
alternating transparent and opaque white rings. These are known as the
halones, and frequently appear at the commencement of incubation. It is
stated by His that they are to be explained by the white-yolk spheres under-
going changes of two kinds. In the one case the spherules they contain are
III.] THE MEDULLARY CANAL. 53
dissolved and give place to vacuoles ; where this occurs to a large extent an
opaque ring is formed. In the other case a solution of the protoplasm of the
spheres takes place, and the spherules are let loose in large numbers ; where
this occurs a transparent ring is formed.
The chief events then of the second part of the first day,
are the appearance of the medullary folds and groove, the
formation of the notochord, the beginning of the head-fold
and amnion, and the histological changes taking place in the
several layers.
7. From the 20th to the 24<th hour. The head-fold en-
larges rapidly, the crescentic groove becoming deeper, while
at the same time the overhanging margin of the groove (the
upper limb of the 8, Chap. II. 5), rises up above the level of
the blastoderm ; in fact, the formation of the head of the
embryo may now be said to have definitely begun.
S. The medullary folds, increasing in size in every di-
mension, but especially in height, lean over from either side
towards the middle line, and thus tend more and more to
roof in the medullary canal, especially near the head.
About the end of the first day they come into direct contact
and completely coalesce with each other at a point which
lies at some little distance behind the head-fold, in the
region which will afterwards become the neck. Union, having
begun at this spot, rapidly runs forward till (early in the
second day) the head-part is completely closed in ; and then
passes more slowly backwards. The whole of the anterior
portion of the groove is closed in before the union has ad-
vanced more than a very short distance towards the tail. In
this way a tubular canal is formed, ending blindly in front,
but as yet open behind. This is the medullary or neural
canal (Fig. 13, M, Fig. 20, Me.). It is not completely
closed in at the tail till a period considerably later than
the one we are considering.
9. Meanwhile important changes are taking place in
the axial portions of the mesoblast, which lie on each side of
the notochord beneath the medullary folds.
In an embryo of the middle period of this day, examined
with transmitted light, the notochord is seen at the bottom
of the medullary groove between the medullary folds, as a
transparent line shining through the floor of the groove when
the embryo is viewed from above. On either side of the
54 THE FIRST BAY. [CHAP.
notochord the body of the embryo appears somewhat opaque,
owing to the thickness of the medullary folds ; as these folds
slope away outwards on either side, so the opacity gradually
fades away in the pellucid area. There is present at the sides
no sharp line of demarcation between the body of the embryo
and the rest of the area ; nor will there be any till the lateral
folds make their appearance ; and transverse vertical sections
shew (Fig. 12) that there is no break in the mesoblast, from
the notochord to the margin of the pellucid area, but only a
gradual thinning.
10. During the latter period of the day, however, the
plates of mesoblast on either side of the notochord begin to
be split horizontally into two layers, the one of which attach-
ing itself to the epi blast, forms with it the somatopleure (Fig.
13, compare also Fig. 20, So.}, while the other, attaching itself
to the hypoblast, forms with it the s/jlanchnopleure (Fig. 13,
Be, Fig. 20, sp). By the separation of these two layers from
each other, a cavity (Fig. 13, pp, and Fig. 20, pp], containing
fluid only, and more conspicuous in certain parts of the
embryo than in others, is developed. This cavity is the be-
ginning of that great serous cavity of the body which after-
wards becomes divided into separate cavities. We shall speak
of it as the pleuro-peritoneal cavity.
11. This cleavage into somatopleure and splanchnopleure
does not extend quite up to the walls of the medullary canal.
Hence there is left along either side of the canal, between it
and the line along which the cleavage begins, a tract or plate
of uncleft mesoblast, which receives the name of vertebral
plate, the more external mesoblast being called the lateral
plate.
At first each vertebral plate is not only unbroken along
its length but also continuous at its outer edge with the
upper and lower layers of the lateral plate of the same side.
Very soon, however, clear transverse lines are seen, in surface
views, stretching inwards across each vertebral plate from the
lateral plate towards the notochord ; and not long after a
transparent longitudinal line makes its appearance on either
side of the notochord along the line of junction of the lateral
with the vertebral plate.
These transparent lines are caused by the appearance of
vertical clefts, giving rise to narrow spaces containing nothing
III.]
THE PROTOVERTEBR.E.
55
but clear fluid ; and transverse sections shew that they are
due to breaches of continuity in the mesoblast only, the
epiblast and hypoblast having no share in the matter. The
first transverse lines which appear are two in number, one a
little behind the other, about opposite the spot where the
medullary folds first coalesced to form the neural tube. The
longitudinal lines begin at about the same place and run thence
backward, parallel to the notochord, as far as the closure of
the medullary canal extends. Behind the first two transverse
lines other parallel transverse lines in course of time make
their appearance.
Thus each vertebral plate appears in surface views to be
cut up into a series of square plots, bounded by transparent
lines. Each square plot is the surface of a corresponding
cubical inass {Fig. 13, P. v., Fig. 20, P. v.). The two such
FIG. 13,
\Ch
'Ao
TRANSVERSE SECTION THROUGH THE DORSAL KEG ION OF AN~ EMBRYO OP THE
SECOND DAT. (Copied from His), introduced here to illustrate the forma-
tion of the protovertebrse, and the cleavage of the mesoblast.
The provertebrse appear irregularly quadrate ; they would have been more
distinctly square, had the section been one of the fi' - st day, before the appear-
ance of the primitive aortse, and of the rudiments of the Wolffiari ducts : com-
pare Fig. 20.
M. medullary canal. Pv. protovertebra. w. rudiment of Wolffian duct.
A. epiblast. C. hypoblast. Ch. notochord. Ao. aorta. BC. splanch-
nopleure.
cubical masses first formed, lying one on either side of the
notochord beneath, and a little to the outside of the medul-
lary folds, are the first pair of protovertebrce. Behind this
first pair, but otherwise similarly situated, a second and third
pair make their appearance during the first day.
56 THE FIRST DAY. [CHAP.
The vertebral plate, while still continuous with the lateral plate (the dis-
tinction between the two being indicated solely by the cleavage of the latter),
consists of several layers of cells ; but of these only the uppermost layer, that
immediately under the epiblast, appears to be continued into the somatopleure ;
the whole of the remainder, including those cells which will eventually form the
so-called nucleus of the protovertebrae, seem to pass directly into the splanchno-
pleure.
All these changes except the formation of the pleuro-peri-
toneal cavity can be seen in surface views of fresh trans-
parent specimens, but their nature is best shewn in sections.
12. Since the commencement of incubation the area
opaca has been spreading outwards over the surface of the
yolk, and by the end of the first day has reached about the
diameter of a sixpence. It appears more or less mottled
over the greater part of its extent, but this is more particu-
larly the case with the portion lying next to the pellucid
area ; so much so, that around the pellucid area an inner ring
of the opaque area may be distinguished from the rest by
the difference of its aspect.
At about the 20th 24th hours an increasing number of
formative cells make their way from the segmentation-cavity
to the edge of the area opaca, and there, immediately under-
neath the epiblast, quickly become converted into a rather
thick and somewhat irregular network of mesoblast cells. The
mottled appearance of the inner ring spoken of above is due
to changes taking place in this mass of mesoblast, changes
which eventually result in the formation of what is called the
vascular area, the outer border of which marks the extreme
limit to which the mesoblast extends.
During the whole of this period the medullary groove has
been growing rapidly backwards, so that the primitive groove
appears to be pushed further and further back, and at the
same time becomes smaller and less conspicuous. The
amniotic fold is at the end of the first day very noticeable.
13. The changes then which occur during the first day
may thus be briefly summarized :
(1) The hypoblast and mesoblast are formed from the
segmentation-spheres, so that by the 6th to the 8th hour the
three layers of the germ the epiblast , the mesoblast , and the
hypoblast are definitely established.
(2) The primitive streak is formed by a thickening of
the mesoblast.
III.] SUMMARY. 57
(3) The primitive groove is formed along the centre of
the primitive streak.
(4) The pellucid area becomes pear-shaped, the broad
end corresponding with the future head of the embryo. Its
long axis lies at right angles, to the long axis of the egg.
(5) The medullary groove makes its appearance in front
of the primitive groove, and below it the notochord is formed
out of mesoblastic cells.
(6) The development of the head-fold gives rise to the
first definite appearance of the head.
(7) The medullary folds rise up and coalesce in the
region of the neck to form the neural tube, the primitive
streak and groove disappearing.
(8) One or more pair of protovertebrce make their ap-
pearance.
(9) By the cleavage of the mesoblast, the somatopleure
separates from the splanchnopleure.
(10) The first trace of the amnion appears in front of
the head-fold.
(11) The vascular area begins to be be distinguished
from the rest of the opaque area.
It may be well to remark, before passing on to the second
day, that out of the protovertebras are formed not only the
permanent vertebrae, but also the superficial dorsal as well
as certain other muscles and the spinal nerves ; that the pair
of protovertebras first formed corresponds not with the first
cervical vertebra of the adult chick, but rather with the third
or even fourth ; for though the majority of the protovertebra3
are formed regularly behind the first pair, two or even three
pair may make their appearance in front of it; and lastly,
that in the part of embryo which forms the head, the meso-
blast is never cut up into proto vertebras, and never under-
goes cleavage to form somatopleure and splanchnopleure.
CHAPTER IV.
THE CHANGES WJIICH TAKE J>I,ACE DURING THE SECOND
DAY.
1. The First Half of the Second Day. In attempting
to remove the blastoderm from an egg which has undergone
from 30 to 36 hours' incubation, the observer cannot fail. to
notice a marked change in the consistency of the blastodermic
structures. The excessive delicacy and softness of texture
which rendered the extraction of an 18 or 20 hours' blasto-
derm so difficult, has given place to a considerable amount of
firmness; the outlines of the embryo and its appendages are
much bolder and more distinct ; and the whole blastoderm
can be removed from the egg with much greater ease,
In the embryo itself viewed from above one of the fea-
tures which first attracts attention is the progress in the
head-fold (Fig. 15). The upper limb or head has become
much more prominent, while the lower groove is not only
proportionately deeper, but is also being carried back beneath
the body of the embryo (Chap. n. 5.)
2. The medullary folds are closing rapidly. In the region
of the head they have quite coalesced, a slight notch in the
middle line at the extreme front marking for some little time
their line of junction. The open medullary groove of the first
day has thus become converted into a tube, the neural canal,
closed in front, but as } 7 et open behind. For a brief period
the calibre of this tube is uniform throughout; but very
speedily the front end dilates into a small bulb, whose cavity
remains continuous with the rest of the neural canal, and
whose walls, like those of the canal, are formed of epiblast.
This bulb is known as the first cerebral vesicle, Fig. 14, FB,
CHAP. IV.]
THE CEREBRAL VESICLES.
59
AN EMBRYO CHICK OF THE FIRST DAT (ABOUT THIRTY-SIX HOURS) VIEWED
FROM BELOW AS A TRANSPARENT OBJECT.
pi. Outline of the pellucid area.
FB. The forebrain or first cerebral vesicle, projecting from the sides of which
are seen the optic vesicles op.. A definite head is now constituted, the
backward limit of the somatopleure fold being indicated by the faint
line S. 0. Around the head are seen the two limbs of the amniotic head-
fold : one, the true amnion a, closely enveloping the head, the other, the
false amnion a', at some distance from it. The head is seen to project
beyond the anterior limit of the pellucid area.
The splanchnopleure fold extends as far back as sp. Along its diverging
limbs are seen the conspicuous venous roots or omphalo-mesaraic veins, uniting
to form the heart h, which continuing forward as the bulbus arteriosus ba, is
lost in the substance of head just in front of the somatopleure fold.
Lying (in this position of the embryo) under the heart is seen the broad
foregut d, the wide crescentic opening into which at the hind limit of the
splanchnopleure fold is very conspicuous. Beneath the foregut are faintly seen
the hind brain tiB. and higher up and more distinctly the mid brain MB.
These are not yet completely differentiated, and their limits are in consequ ence
very obscurely indicated.
GO THE SECOND DAY. [CHAP.
Behind the splanchnopleure fold, marking the hind limits of the foregut,
are seen the two rows of proto vertebrae, the dark line between which m. c.
indicates the position both of the line of junction of the medullary folds and of
the notochord. The front end of the notochord is seen at ch. underneath the
forebrain; its hind end is indistinct. Towards the tail the proto vertebrae
become indistinct and give place to the vertebral plates v. pi. Still further
back, at the commencing tail, all the parts become indistinct, the remains of
the primitive groove pv. being as conspicuous as anything else.
and makes its appearance in the early hours of the second
day. Behind it a second and a third bulb, the second and
third cerebral vesicles, are successively formed in a similar
manner; but the consideration of these, though they begin
to make their appearance soon after the formation of the
first cerebral vesicle, may be conveniently reserved to a later
period.
3. The number of proto vertebras increases rapidly.
The one or two pairs which are seen at the end of the first
day have by the middle of the second day multiplied to five,
or eight, or even more, Figs. 14, 15, p.v, each being formed in
the same way as the first. As was mentioned previously,
the chief increase takes place from before backwards, the
new protovertebras appearing behind the old ones; but one
pair at least is probably formed in front of that which was
the very first to appear.
In the early part of this day the formation of new proto-
vertebras keeps pace with the closing in of the medullary folds,
so that that part of the canal which is already closed in is
always flanked by protovertebras ; but later on the formation
of protovertebrge lags behind, so that for some distance to-
wards the hinder extremity the closed medullary canal is
unprotected by proto vertebras, Fig. 15. At the extreme end
the medullary folds become shallower, diverge from each
other, and afterwards meet again, thus forming a lozenge-
shaped open depression known as the sinus rhombpidalis,
Fig. 15, s. r.
Behind the sinus rhomboidalis there may generally be seen a small and
usually curved remnant of the primitive groove. Fig. 15, p. r.
4. In a former chapter it was pointed out (Chap. II. 5)
that the embryo is virtually formed by a folding or tucking
in of the limited portion of the blastoderm, first at the anterior
extremity, and afterwards at the posterior extremity and at
IV.]
THE ALIMENTARY CANAL.
FIG. 15.
61
EMBRYO OF THE CHICK AT 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE
OBJECT. (Chromic acid preparation.)
/. b. front brain, m. b. mid brain. Ti. b. hind brain, op. v. optic vesicle.
au. p. auditory vesicle, o. f. omphalo-mesaraic vein. p. v. protovertebra.
m. f. line of junction of the medullary folds above the medullary canal.
8. r. sinus rhomboidalis. t. tail-fold, p. r. remains of primitive groove.
a. p. area pellucida.
The line to the side between p. v. and m. f. represents the true length
of the embryo.
The biscuit-shaped outline indicates the margin of the pellucid area. The
head, which reaches as far back as o.f, is distinctly marked off ; but neither the
somatopleuric nor splanchnopleuric folds are shewn in the figure ; the latter
diverge at the level of o.f, the former considerably nearer the front, somewhere
between the lines m. b. and h. b. The optic vesicles op. v. are seen bulging
out beneath the superficial epiblast. The heart lying underneath the opaque
body cannot be seen. The tail-fold, t., is just indicated ; no distinct lateral
folds are as yet visible in the region midway between head and tail. At m. /.
the line of junction between the medullary folds is still visible, being lost
forwards over the cerebral vesicles, while behind the folds diverge to enclose
the narrowing sinus rhomboidalis, s. r.
62
THE SECOND DAY.
[CHAP.
the sides. One of the results of this doubling up of the blasto-
derm to form the head is the appearance, below the anterior
extremity of the medullary tube, of a short canal, ending
blindly in front, but open widely behind (Fig. 16, D], a cut de
sac in fact, lined with hypoblast reaching from the extreme
front of the embryo to the point where the splanchnopleuric
leaf of the head-fold (Fig. 16, F. Sp) turns back on itself.
This cul de sac, which of course becomes longer and longer
the farther ba.ck the head-fold is carried, is the rudiment of
the front end of the alimentary canal, the foregut, as it might
be called. In transverse section it appears to be flattened
horizontally, and also bent, so as to have its convex surface
looking downwards, (Fig. 18 al). At first the anterior end is
quite blind, there being no mouth at all ; the formation of
this at a subsequent date will be described later on.
At the end of the first half of the second day the head-
FIG. 1 6.
N.C.
F.So.
DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF AN EMBRYO.
The section is supposed to be made at a time when the head -fold has oom^
menced but the tail-fold has not yet appeared.
N. C. neural canal, closed in front but as yet open behind. Ck. notochord, not
reaching to the extreme front, and not as yet fully formed behind. The
section being taken in the middle line, the protovertebrse are of course not
shewn. In front of the notochord is seen a mass of un cleft mesoblast,
which will eventually form part of the skull. D. the commencing fore-
gut or front part of the alimentary canal. F. So. Somatopleure, raised
up in its peripheral portion into the amniotic fold Am. Sp. Splanchno-
pleure. At Sp. it forms the under wall of the foregut; at F. Sp. it is turning
round and about to run forward. Just at its turning point the cavity of the
heart lit is being developed in its mesoblast. pp. pleuroperitoneal cavity.
A epiblast, B mesoblast, C hypoblast, indicated in the rest of the figure by
differences in the shading. At the part where these three lines of reference
end the mesoblast is as yet uncleft.
IV.] THE HEART. 63
fold has not proceeded very far backwards, and its limits
can easily be seen in the fresh embryo both from above and
from below.
5. It is in the head-fold that the formation of the heart
takes place, its mode of origin being connected with that
cleavage of the mesoblast and consequent formation of splanch-
nopleure and somatopleure of which we have already spoken.
At the extreme end of the embryo (Fig. 16), where the
blastoderm begins to be folded back, the mesoblast is never
cleft, and here consequently there is neither somatopleure nor
splanchnopleure ; but at a point a very little further back,
close under the blind end of the foregut, the cleavage (at
the stage of which we are speaking) begins, and the somato-
pleure, F. So, and splanchnopleure, F. Sp. diverge from
each other. They thus enclose between them a cavity, pp,
which rapidly increases behind by reason of the fact that
the fold of the splanchnopleure is carried on towards the
hinder extremity of the embryo considerably in advance of
that of the somatopleure. Both folds, after running a certain
distance towards the hind end of the embryo, are turned
round again, and then course once more forwards over the
yolk-sac. As they thus return (the somatopleure having
meanwhile given off the fold of the ammion, Am.), they are
united again to form the uncleft blastodermic investment of
the yolk-sac. In this way the cavity arising from their sepa-
ration is closed below.
It is in this cavity, which from its mode of formation the
reader will recognise as a part (and indeed at this epoch it
constitutes the greater part) of the general pleuroperitoneal
cavity, -that the heart is formed.
It makes its appearance at the under surface and hind
end of the foregut just where the splanchnopleure folds turn
round to pursue a forward course, (Fig. 16, Ht)\ and by the
end of the first half of the second day (Fig. 14, h) has ac-
quired somewhat the form of a flask with a slight bend to
the right. At its anterior end a slight swelling marks the
future bulbus arUriosus ; and a bulging behind indicates the
position of the auricles. It is hollow, and its cavity opens
below and behind into two vessels called the omphalo-mesaraic
veins (Figs. 14, 15, o.f), which pass outwards in the folds of the
splanchnopleure at nearly right angles to the axis of the
64 THE SECOND DAY. [CHAP.
embryo. The anterior extremity of the heart is connected
with the two aortse.
The muscular portion of the walls of the heart are derived in the chick (as
in all other vertebrates in which the point has been worked out) from the
mesoblast of the splanchnopleure.
Although thus much may be asserted with tolerable certainty for all verte-
brates; yet the exact mode of development appears, according to our present
knowledge, to be very different in different cases ; and it seems probable that
these differences are in part the result of variations in the mode of formation
and time of closure of the alimentary canal.
In the chick the investigation of the earlier stages of the heart is beset with
considerable difficulties; and accordingly various inquirers have arrived at very
different results, though the majority are agreed as to its formation from the
mesoblast of the splanchnopleure. Exact information concerning the epithe-
lium lining the heart may be said to be almost completely wanting.
Von Baer described the heart as consisting in its earliest stage of two solid
aggregations of the mesoblast cells of the splanchnopleure, converging in front
at the end of the foregut where they are loosely united together by a thin band,
but diverging behind along the diverging folds of the splanchnopleure. As the
foregut lengthens, the two masses coalesce more and more completely in front,
until the whole structure assumes the shape of a fusiform mass, attached to the
under wall of the foregut, with prolongations stretching like the limbs of
an inverted & along the folds of the splanchnopleure on either side. A t first
solid throughout, the ^-shape mass subsequently becomes hollowed out and
filled with fluid by the solution of its central cells.
The account given by Eemak (EntwicTcelung der Wirbelthiere, 1855) is some-
what similar.
According to His the heart is formed by the separation of a layer of the
splanchnopleure and its coalescence with a similar layer from the somatopleure.
It is therefore from the beginning hollow. Its cavity is also from the be-
ginning continuous with the canals of the aortae and omphalo-mesaraic veins,
the roots of which are formed in a precisely similar manner as itself. It is
through these that the epithelial (endothelial) elements, derived from the white
yolk, make their way into the heart to form its epithelial (endothelial) lining.
According to Afanassieff (Bull. Acad. St. Petersbourg., Tom. xm. 1869,
pp. 221 335) the heart is formed by the longitudinal separation of a thick
layer of the mesoblast of the splanchnopleure along the under wall of the fore-
gut. At either side, so much of the mesoblast is detached, that only a single
layer of cells (seen spindle-shaped in a transverse section of the embryo) remains
united with the hypoblast to form the wall of the gut. Along the middle line
the separation is not complete, the detached layer of mesoblast being here still
connected with the wall of the gut by a few cells. The single layer of spindle-
shaped cells breaks loose in turn from the wall of the gut on either side of the
mill die line, but still remains attached along the middle line itself. We have
thus in transverse section a thinner and a thicker layer of mesoblast, hanging
down in a double festoon from the (hypoblastic) under-wall of the gut. Both
layers become more and more separated from the gut, and bulge out into the
pleuroperitoneal space, thus creating between themselves and the gut a cavity,
which is at first double, but, by the disappearance of the cells along the middle
line, subsequently becomes single. This is the cavity of the heart, the thick
layer representing its muscular walls, and the thin its epithelial lining. The
two ends are open, the hinder end being connected with the omphalo-mesaraic
veins, and the front with the aortse. At first the heart is not a tube with
IV.] THE HEART. 65
complete walls of its own, but rather a cavity, closed in below and at the sides
by its mesoblantic walls, and roofed over by the bare hypoblastic under-wall of
the foregut. Very shortly, however, the side walls close in above, and thus
pinch off the heart as a complete and distinct tube, which becomes quite
detached along the greater part of its length from the wall of the gut, though it
still remains connected with it, both at the venous and arterial ends.
Klein (Wien. Sitzungsbericht, LXIII. n., iSyr) considers that the heart
is formed from the cells of the mesoblast of the splanchnopleure as a body
which, at first solid, subsequently becomes hollow by the conversion of its
central cells into blood-corpuscles. The layer of cells immediately surrounding
the blood-corpuscles forms the epithelial lining, and subsequently becomes
connected with that of the great arteries and veins.
The following view, which our own observations have led us to adopt,
agrees with that of Klein in regarding the heart as being at first a solid thicken-
ing of the mesoblast of the splanchnopleure ; but its accordance with the earlier
Statements of Von Baer is much more complete.
In order to understand the formation of the heart it must be distinctly borne
in mind that in the region where the heart is about to appear, the splanch-
nopleure is continually being folded in on either side, and that these lateral
folds are progressively meeting and uniting in the middle line to form the
under-wall of the foregut (that which in the adult chick will be the anterior
wall of some portion of the alimentary canal). (Compare Chap. n. 5.)
At any given moment these folds will be found to have completely united
in the middle line along a certain distance measured from the point in front
where the cleavage of the mesoblast (i.e. the separation into somatopleure
and splanchnopleure) begins, to a particular point farther back.
At this particular point the folds will have met, but not united, Fig. 17, A.
Further back still they will have not even met, but will appear simply inclined
towards each other, Fig. 17, B. Or, to put it in another way, they will here be
found to be diverging from the point where they were united, and not only diverg-
ing laterally each from the middle line, but also both turning so as to run in
a forward direction to regain the surface of the yolk and rejoin the somato-
pleure, Fig. 1 6. In a transverse section taken behind this extreme point
of union, or point of divergence, as we may call it, the splanchnopleure on
either side when traced downwards from the axis of the embryo may be
seen to bend in towards the middle so as to approach its fellow, and then
to run rapidly outwards, Fig. 17, B. A longitudinal section shews that it runs
forwards also at the same time, Fig< 16. A section through the very point
of divergence shews the two folds meeting in the middle line and then separating
again, so as to form something like the letter X, with the upper limbs con-
verging, and the lower limbs diverging. In a section taken in front of the
point of divergence, Fig. 18, the lower diverging limbs of the X have disappeared
altogether; nothing is left but the upper limbs, v\hich, completely united in the
middle line, form the under- wall of the foregut.
As development proceeds, what we have called the point of divergence
is continually being carried farther and farther back, so that the distance
between it and the point where the somatopleure and splanchnopleure separate
from each other in front, i.e. the length of the foregut, is continually increasing.
When the heart is about to be formed, thickenings are observed in the
mesoblast of the splanchnopleure, along the diverging folds, i.e. along the
lower limbs of the X just behind the point of divergence. Ihese thickenings
are continued into each other by a similar thickening of the mesoblast ex-
tending through the point of divergence itself.
At first there is no thickening of the mesoblast in front of the point of
divergence, i.e. along the under- wall of the foregut. As the point of diverg-
E. 5
06 THE SECOND DAY. [CHAP.
ence however is in the Course of events carried farther back, though the
lower diverging folds (the lower limbs of the X) disappear, the thickening at
the point remains and increases. In a short time, consequently, we do find
a thickening of the mesoblast in the under-wall of the foregut just in front
of the point of divergence, which thickerting is continuous like an inverted j^,
with two thickenings reaching down the diverging folds behind the point
of divergence.
This j^-shaped thickening becomes hollow by a transformation of its
central cells ; the single cavity in front is the cavity of the heart, and the two
diverging cavities behind, with which it is continuous, are the canals of the
omphalo-mesaraic veins.
As development proceeds, and the point of divergence is carried still farther
and farther back, the heart increases in length step by step at the expense
of the continually coalescing omphalo-mesaraic veins.
The coalescence of the mesoblastic thickening which forms the walls of the
veins precedes that of their canals, consequently in sections taken at parti-
cular points we meet with two cavities invested by one wall. This is probably
what was seen by the observers who have described the heart as being formed
as a double tube which afterwards became single.
The front end of the cavity of the heart is continuous with canals similarly
formed in the mesoblast of the foregut by the solution of certain ceils. These
are the canals of the aortse.
At first the substance of the heart is along its whole length adherent to and
indeed a part of the underwall of the foregut. Subsequently it becomes free in
its middle portion, the arterial and venous ends alone remaining attached.
Soon after its formation the heart begins to beat, its at
first slow and rare pulsations beginning at the venous and
passing on to the arterial end. It is of some interest to
note that its functional activity commences long before the
cells of which it is composed shew any distinct differentiation
into muscular or nervous elements.
6. To provide channels for the fluid thus pressed by the
contractions of the heart, a system of tubes has made its
appearance in the mesoblast both of the embryo itself and of
the vascular and pellucid areas. In front the single tube of
the heart bifurcates into two primitive aortce, each of which
bending round the front end of the foregut, passes from its
under to its upper side, the two forming together a sort of
incomplete arterial collar imbedded in the mesoblast of the
gut. Arrived at the upper side of the gut, they turn sharply
round, and run separate but parallel to each other backwards
towards the tail, in the mesoblast on each side of the notochord
immediately under the protovertebra (Figs. 18, Ao, 20, Ao).
About half way to the hinder extremity each gives off at right
angles to the axis of the embryo a large branch, the omphalo*
mesaraic artery (Fig. 23, Of, A.}, which, passing outwards, is
IV.]
THE BLOOD-VESSELS.
67
distributed over the pellucid and vascular areas, the main
trunk of each aorta passing on with greatly diminished calibre
towards the tail, in which it becomes lost.
FIG. 17. A.
FIG. 17. B,
Tib,
<*.?
TWO CONSECUTIVE SECTIONS OF A 36 HOURS' EMBRYO ILLUSTRATING THE
FORMATION OF THE HEART. A IS THE MOST ANTERIOR OF THE TWO.
h. b. hind brain. nc. notochord. E. epiblast. so. somatopleure. sp.
splanchnopleure. d. alimentary canal, hy. hypoblast. hz. (in A) heart.
of. omphalo-mesaraic vein.
The heart is seen from the sections to be formed from the mesoblast of the
splanchnopleure. It is not however split off from a portion of the mesoblast
which forms the muscular wall of the alimentary canal, but the mesoblast, where
it turns round to run outwards again over the yolk-sack, becomes thickened,
and in each of the thickenings (one on each side) so formed, a cavity appears
52
68 THE SECOND DAY. [CHAP.
forming immediately behind'the heart the omphalo-mesaraic veins (section J?), (of).
As however the folding of the splanchnopleure becomes more complete, and the
digestive canal becomes completely closed (instead of remaining partially open
as in section B), these two cavities unite; and an appearance is produced
similar to that represented in figure A, where there is the single cavity of the
heart (hz). In the interior of the heart is seen a lining of flattened cells.
The shading, as will be seen, is purely diagrammatic. The epiblast,
whether superficial as at E, or involuted as part of the neural canal hb, is
shaded of one tint. The mesoblast, whether uncleft, or diverging into soma-
topleure and splanchnopleure, is of another tint. In the hypoblast a distinction
has been drawn between the thickened portion which lines the alimentary
canal, and the thinner portion which belongs to the more peripheral part
of the splanchnopleure, the two being at first continuous as in B, and afterwards
separated as in A.
It will be understood that the two figures, though actually two consecutive
sections of the same embryo, may be taken to represent two phases of the
formation of the heart. B in the process of development will become A, and
A a short time previously was in the condition of B.
TRANSVERSE SECTION OF AN EMBRYO AT THE END OF THE SECOND DAY PASSING
THROUGH THE REGION OF BULBUS ARTEttlOSUS. (Copied from His.)
M. medullary canal in the region of the hind brain. V. anterior cardinal or
superior vertebral vein. Ao. Aorta. Ch. Notochord. al. alimentary
canal. H. Heart (bulbus arteriosus). Pp. Pleuroperitoneal cavity.
am. amnion.
On comparing this with Fig. 17, it will be seen that the mesoblast (muscular)
wall of the heart H has now become quite separate from the rest of the
mesoblast of the splanchnopleure, which forms, in the section, an independent
line below the heart, the section of branches of the omphalo-mesaraic veins
being seen on either side. The bridle of mesoblast represented in the drawing as
passing from the splanchnopleure below to the sounatopleure above, reaching the
latter just inside the fold of the anmion, has been described by His, but has
never been seen by ourselves.
In the vascular and pellucid areas, the formation of
vascular channels with a subsequent differentiation into
arteries, capillaries and veins, is proceeding rapidly. Blood-
IV.] THE BLOOD-VESSELS. 69
corpuscles too are being formed in considerable numbers.
The mottled yellow vascular area becomes covered with red
patches consisting of aggregations of blood-corpuscles, often
spoken of as blood-islands.
Round the extreme margin of the vascular area and nearly
completely encircling it, is seen a thin red line, the sinus or
vena, terminal-is (Fig. 23, Sv.). This will soon increase in
size and importance.
From the vascular and pellucid area several large channels
are seen to unite and form two large trunks, one on either
side, which running along the splanchnopleure folds at nearly
right angles to the axis of the embryo, unite at the " point of
divergence" to join the venous end of the heart. These are
the omphalo-mesaraic veins (Figs. 14, o.f., 23, o.f.) spoken of
above.
Both vessels and corpuscles are formed entirely from the
cells of the mesoblast; and in the regions where the meso-
blast is cleft, are at first observed exclusively in the splanch-
nopleure. Ultimately of course they are found in the meso-
blast everywhere.
The mode of formation of the blood-vessels and corpuscles has been much
and long debated. The observations of one of us have led us to believe the
following to be the true account.
In the pellucid area, where the formation of blood-vessels may be most
easily observed, a number of mesoblastic cells are seen to send out processes.
These processes unite, and by their union a protoplasmic network is formed
containing nuclei at the points from which the processes started. The nuclei,
which as a rule are much elongated and contain large oval nucleoli, increase
very rapidly by division, and thus form groups of nuclei at the, so to speak,
nodal points of the network. Several nuclei may also be seen here and there
in the processes themselves. The network being completed, these groups, by con-
tinued division of the nuclei, increase rapidly in size; the majority of the nuclei
composing them acquire a red colour and become converted into blood-corpuscles
(Fig. 19, b.c.) ; but a few, generally on the outside of the group, remain un L
unaltered, (Fig. 19, a}. The protoplasm in which the central reddened nuclei
are imbedded becomes liquefied, while that on the outside of each group, as well
as that of the uniting processes, remains granular, and increasing in quantity,
forms an investment for the unaltered nuclei which are embedded in it.
Each nodal point is thus transformed into a more or less rounded mass of
blood-corpuscles floating in plasma but enveloped by a layer of nucleated proto-
plasm, the several groups being united by strands of nucleated protoplasm. These
uniting strands rapidly increase in thickness; new processes are also continually
being formed ; and thus the network is kept close and thickset while the area is
increasing in size.
JBy a transformation of nuclei similar to that which took place in the nodal
points, blood-corpuscles make their appearance in the processes also, the central
portions of which become at the same time liquefied. The uncoloured nuclei
70
THE SECOND DAY.
[CHAP.
situate in the envelopes of the nodal groups, as well as those lying on the exterior
of the connecting processes, appropriate a quantity of the granular protoplasm
surrounding each, and thus become converted into spindle-shaped cells. Each
nodal group and each connecting process thus gets a distinct wall of nucleated
cells. By the continued widening of the connecting processes and solution of their
central portions, accompanied by a corresponding increase in the enveloping
nucleated cells, the original protoplasmic network is converted into a system
of communicating tubes, the canals of which contain blood-corpuscles and
plasma, and the walls of which are formed of spindle-shaped nucleated cells.
FIG. 19.
I.e.-
SURFACE VIEW FROM BELOW OF A SMALL PORTION OF THE POSTERIOR END OF
THE PELLUCID AREA OF A 36 HOURS' CHICK. To illustrate the formation of
the blood- capillaries and blood-corpuscles, magnified 400 diameters.
6. c. Blood- corpuscles at a nodal point, already beginning to acquire a red
colour. They are enclosed in masses of protoplasm in the outermost
layer of which are found nuclei, a, some of which contain two nucleoli. These
nuclei subsequently become the nuclei of the cells forming the walls of
the vessels. The nodal groups are united by protoplasmic processes (p.pr),
also containing nuclei with large nucleoli (ri). These nuclei increase in
number by division, and become converted in part into the nuclei of the
cells forming the walls of the vessels, and in part into blood-corpuscles.
The blood-corpuscles pass freely from the nodal points into the hollow pro-
cesses, and thus the network of protoplasm becomes a network of blood-vessels ;
the corpuscles and the nuclei of the walls of which have been by separate
paths of development derived from the nuclei of the original protoplasm.
The formation of the corpuscles does not proceed equally rapidly or to the
JV.] THE BLOOD-VESSELS. 71
same extent in all parts of the blastoderm. By far the greater part are formed
in the vascular area, but some arise in the pellucid area, especially in the hinder
part. In the front of the pellucid area the processes are longer and the network
accordingly more open ; the corpuscles also are both later in appearing and
less numerous when formed.
The omphalo-mesaraic arteries and veins, and the sinus terminalis which
from the first has a distinct wall, seem to take origin in a manner altogether
similar to that of the smaller vessels ; and the description of the formation of
the heart which we gave above shews that it too is nothing but a gigantic
nodal point.
Assuming the truth of the above account, it is evident that the blood-vessels
of the chick do not arise as spaces or channels between adjacent cells of the
mesoblast, but are hollowed out in the communicating protoplasmic substance of
the cells themselves. It is also perhaps worthy of note that the red-blood
corpuscles are not cells, but nuclei.
The red- blood corpuscles when removed from the vessels exhibit energetic
amoeboid movements. They seem to increase at this stage chiefly by division.
The above is the view which we deduce from our own observations. The
following may serve as a brief summary of the history of the matter.
Von Baer and the older embryologists regarded the blood-vessels as being
at first mere gaps or spaces between the cellular elements of the mesoblast,
hollowed out so to speak by the now of blood from the heart. The first steps
in the right direction were taken by Remak and IQ>lliker, who described the
formation of solid bands or cylinders composed of cells and arranged in a
close-set network. These bands, becoming hollowed by solution while their
central cells were converted into blood-corpuscles, gradually put on the
appearance of blood-vessels, the aggregation of the red corpuscles at various
points, through arrest of the circulation, giving rise to the blood-islands of Wolfl
and Pander.
According to Af an assieff ( Wien. Sitz. Bericht. Bd. 53, 1866) there appear
in the mesoblast vesicles of variable size, with protoplasmic envelopes and
contents. These vesicles, which are at first clear and homogeneous, subse-
quently become traversed with strands of nucleated protoplasm, forming often a
close net-work within the vesicle. The space intervening between the nume-
rous vesicles is cut up into a network of canals by protoplasmic processes
stretching from one vesicle to another. These canals are the rudimentary
blood-vessels. From the outside of the vesicles, forming the inner wall of the
adjacent vessels, nucleated masses of protoplasm are budded off as blood-cor-
puscles and fall into the current of the circulation.
His (op. cit.), following out his peculiar theory of development, derived both
blood and blood-vessels from the white yolk or parablast. According to him
while certain of the white-yolk masses become converted into conglomerations
of cells, which acquiring a yellow colour stand out in suri'ace views as blood-
islands, other white-yolk masses, metamorphosed into angular cells, form a
network of thick lines permeating the mass of true blastodermic (archiblastic)
cells of the mesoblast. These lines, at the first solid, subsequently become
hollow. The meshwork of canals, or rudimentary blood-vessels, thus developed
first in the vascular and pellucid areas and spreading thence into the embryo,
contains for a certain time clear fluid only, the blood-islands being imbedded
in or attached to the walls of the canal and surrounded by protoplasmic
envelopes, so that the blood-corpuscles are shut out from the cavities of the
vessels. Later on, however, the envelopes of the islands are broken through,
and the blood -corpuscles emerging from their nests fall into the current of the
circulation.
His therefore regarded the blood- corpuscles as formed in greater part at least
72 THE SECOND DAY. [CHAP.
separately from the blood-vessels ; their entrance into the vascular spaces being
an after event. The parablastic cells (derived from the white yolk), in his view,
give rise to the epithelium (endothelium) and connective tissue elements only
of the blood vessels, the muscular elements being derived from genuine (blasto-
dermic) mesoblastic cells.
Klein (Wien. Sitz. Bericht. LXIII. 1871) describes the blood-vessels as
taking their origin from certain cells of the mesoblast in which a vacuole,
appearing and rapidly increasing in size, pushes the nucleus on one side,
leaving only a thin layer of protoplasm round the periphery of the cell. In this
thin layer nuclei appear; and, multiplying, form a complete nucleated invest-
ment, to the vacuole, which meanwhile continues to increase in size. From the
inside of this protoplasmic investment cells are budded off, and fall into the
vacuole. Here they soon acquire a red colour and become converted into
blood-corpuscles. From the exterior of these vacuolated cells nucleated
processes are thrown out, which end freely or join with similar processes from
other cells. A protoplasmic network is thus formed, the lines of which become
vacuolated, and hollow, and ultimately communicate with the original central
vacuoles now crowded with corpuscles. By these means a system of com-
municating tubes is established. Klein also describes two other forms of
cells somewhat differing from the above, but also taking part in the formation
of the blood-vessels. One of these forms is found chiefly in the vascular area,
and he believes that these latter are simply the formative cells of which we
have already so often spoken.
It will thus be seen that Klein's view, from which our own differs chiefly in
reference to the matter of vacuolation, is a return, with some modifications and
extensions, to the earlier view of Rernak, and that the accounts of both Afanas-
sieff and His, which in turn agree in many respects, have proved to be uncorro-
borated divergences from the older track.
Still more recently Goette (Archiv fur Micro. Anat. Vol. X. 1873, pp.
145 199) has given an entirely different account of the origin of the blood-
vessels and bloo<l -corpuscles in the vascular area. He believes that in the
thick mass of cells immediately outside the 'pellucid area' (vide Chap. in. 12)
a quantity of fluid collects and causes the cells to separate into a network with
large spaces filled with fluid. Into these spaces the formative cells travel, and
undergoing a species of endogenous cell-formation, form masses of bl/od-
corpuscles the blood-islands of the ear.ier authors. This view differs, it will
be seen, from all the later views, and goes back to that of Von Baer in regard-
ing the blood-vessels as primitively mere gaps between the cellular elements.
In the investigation of such a point as this, sections (which apparently Goette
has alone employed) are very untrustworthy.
7. The cells of the epiblast and hypoblast as well as of the mesoblast
undergo considerable changes between the 24th and the 36th hour.
Up to the 24th hour the cells of both layers, but more especially of the
hypoblast, were filled with fine granules and also contained many highly
refractive spherules. By the 36th hour, however, they have become much
more transparent. Each cell now consists of a clear protoplasm with hardly
any granules or spherules, and a large oval nucleus together with one or more
vacuoles is distinctly visible.
The cells of the hypoblast still pass insensibly into the white-yolk cells ;
and it is still by the conversion of the white yolk into hypoblast that the
peripheral extension of the latter is chiefly carried on.
The hypoblast cells beneath and at the sides of the embryo are markedly
smaller than those at the periphery of the pellucid area.
The epiblast cells exhibit considerable variation in size in different parts of
IV.] THE WOLFFIAN DUCT. 73
the embryo ; but all are considerably smaller and also somewhat more
columnar than the more peripheral cells of the pellucid area. The largest
epiblastic cells are to be found in the region of the vascular area, but here
they are much flattened. At the extreme outer edge of the opaque area the
cells are smaller again, shewing in this respect a marked contrast to their con-
dition during the previous stage.
8. About this period there may be seen in transverse
sections, taken through the embryo in the region of the proto-
vertebrae, a small group of cells (Fig. 20, W. d) projecting on
either side from the mass of uncleft mesobiast on the outside
of the protovertebrse, into the somewhat triangular space
formed by the epiblast above, the upper and outer angle of the
proto vertebra on the inside, and the mesobiast on the outside.
This group of cells is the section of a longitudinal ridge,
the rudiment of the Wolffian duct. We shall return to it
immediately.
9. The most important changes then which take place
during the first half of the second day, are the closure of the
medullary folds, especially in the anterior part, and the
dilatation of the canal so formed into the first cerebral
vesicle ; the establishment of a certain number of protoverte-
brse ; the elevation of the head from the plane of the
blastoderm ; the formation of the tubular heart and of the
great blood-vessels; and the appearance of the rudiment of
the Wolffian duct. It is important to remember that the
embryo of which we are now speaking is simply a part of the
whole germinal membrane, which is gradually spreading over
the surface of the yolk. It is important also to bear in mind
that all that part of the embryo which is in front of the most
anterior proto vertebrse corresponds to the future head, and
the rest to the neck, body and tail. At this period the head
occupies nearly a third of the whole length of the embryo.
10. The changes which take place from the 36th to
the 45th hour will best form the next stage, since those
which occur during the last few hours of the second day will
be more conveniently described with the third day.
One important feature of the stage is the rapid increase
in the process of the folding off of the embryo from the plane
of the germ, and its consequent conversion into a distinct
tubular cavity. At the beginning of the day, the head alone
projected from the rest of the germ, the remainder of the
[CHAP.
TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO OF
45 HOURS.
A. epiblast. B. mesoblast. C. hypoblast consisting of a single row of flat-
tened cells. M. c. medullary canal. P. v. protovertebra. W. d. Wolffian
duct. S. o. Somatopleure. S. p. Splanchnopleure. p. p. pleuroperitoneal
IV.]
THE TAIL-FOLD.
75
cavity, c. h. Dotochord. a. o. dorsal aorta, v. blood-vessels of the
yolk-sac, o. p. line of junction between opaque and pellucid areas ;
at this point the hypoblast cells are seen to pass without any strong
line of demarcation into the white-yolk spheres, w. white-yolk spheres,
some of which near the edge of the pellucid area contain a body very
like a nucleus.
Only one half of the section is represented in the figure if completed it
would be bilaterally symmetrical about the line of the medullary canal.
*. FIG. 21.
op.v?
EMBRYO OP THE CHICK AT 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE
OBJECT. (Chromic acid preparation.)
/. b. front brain. ra. b. mid brain. h. b. hind brain, op. v. optic vesicle.
au. p. auditory vesicle, o. f. omphalo-mesaraic vein. p. v. protovertebra.
m. f. line of junction of the medullary folds above the medullary canal.
s. r. sinus rhomboidalis. t. tail-fold, p. r. remains of primitive groove.
a. p. area pellucida.
embryo being simply a part of a flat blastoderm, nearly
completely level from the front protovertebrse to the hind
edge of the pellucid area. At this epoch, however, a tail-fold
76 THE SECOND DAY. [CHAP.
(Fig. 21, f) makes its appearance, elevating the tail above the
level of the blastoderm in the same way that the head was
elevated. Lateral folds also, one on either side, soon begin
to be very obvious. By the progress of these, together with the
rapid backward extension of the head-fold and the slower
forward extension of the tail-fold, the body of the embryo
becomes more and more distinctly raised up and marked
off from the rest of the blastoderm.
11. The medullary canal closes up rapidly. The wide
, sinus rhomboidalis becomes a narrow fusiform space (Fig. 21,
s.r.), and at the end of this period is entirely roofed over. The
conversion of the original medullary groove into a closed tube
is thus completed.
12. In the region of the head most important changes
now take place. We saw that at the beginning of this day
the front end of the medullary canal was dilated into a buib,
the first cerebral vesicle. This, from the very first broader
than long, now increases so much in breadth as to give the
embryo a hammer-headed appearance. The lateral portions,
continuing to enlarge, become after a while separated by
constrictions from the central portion. The single vesicle
is thus converted into three vesicles : a median one connected
by short hollow stalks with a lateral one on either side. The
lateral vesicles are known as the optic vesicles (Fig. 21, op. v,
Fig. 22, a), and will afterwards become converted into parts
HEAD OP A CHICK AT THE END OP THE SECOND DAT VIEWED FROM BELOW
AS A TRANSPARENT OBJECT. (Copied from Huxley).
/. first cerebral vesicle, a. optic vesicle, d. infundibulum.
The specimen shews the formation of the optic vesicles (a), as outgrowths
from the ist cerebral vesicle or vesicle of the 3rd ventricle, so that the optic
vesicles and vesicle of the 3rd ventricle at first freely communicated with each
other, and also the growth of the lower wall of the vesicle of the 3rd ventricle
into a process which becomes the infundibulum (d).
IV.] THE OPTIC VESICLES. 77
of the eyes ; the median one still retains the name of the first
cerebral vesicle. The constriction takes place chiefly from
above downwards, so that the optic vesicles soon appear to
spring from the under portions of the cerebral vesicle.
The original vesicle being primarily an involution of the
epiblast, the walls of all three vesicles are formed of epiblast ;
all three vesicles are likewise covered over with the common
epiblastic investment which will eventually become the
epidermis of the skin of the head. Between this superficial
epiblast and the involuted epiblast of the vesicles, there
exists a certain quantity of mesoblast to serve as the material
out of which will be formed the dermis of the scalp, the
skull, and other parts of the head. At this epoch, however,
the mesoblast is found chiefly underneath the several vesicles.
A small quantity may in section be seen at the sides ; but
at the top the epidermic epiblast is either in close contact with
the involuted epiblast of the cerebral and optic vesicles or
separated from it by fluid alone, there being as yet in this
region no cellular elements between the two representing
the mesoblast.
The constrictions marking off the optic vesicles take
place of course beneath the cjmmon epiblastic investment,
which is not involved in them. As a consequence, though
easily seen in the transparent fresh embryo (Fig. 22), they
are but slightly indicated in hardened specimens (Fig. 21).
In sections they are very clearly seen.
13. When an embryo of the early part of the second day
is examined as a transparent object, that portion of the
medullary canal which lies immediately behind the first
cerebral vesicle is seen to be conical in shape, with its walls
thrown into a number of wrinkles. These wrinkles may
vary a good deal in appearance, and shift from time to time,
but eventually, before the close of the second day, after the
formation of the optical vesicles, settle down into two con-
strictions, one separating the first cerebral vesicle from that
part of the medullary canal which is immediately behind
it, and the other separating that second portion from a third.
So, instead of there being one cerebral vesicle only, as at the
commencement of the second day, there is now, in addition
to the optic vesicles, a series of three, one behind the other ;
a second and third cerebral vesicle have been added to the
78 THE SECOND DAY. [CHAP.
first (Fig. 21, ml, hb). They may be also called the "fore
brain," the " mid brain," and the " hind brain," for into these
parts will they eventually be developed.
14. The optic vesicles, lying underneath the epiblast,
towards the end of the day are turned back and pressed
somewhat backwards and downwards against the sides of
the first cerebral vesicle or fore brain, an elongation of
their stalks permitting this movement to take place. The
whole head becomes in consequence somewhat thicker and
rounder.
15. Before the end of the day the fore brain, by a pro-
cess similar to that whereby the optic vesicles were formed,
viz. undue growth followed by constriction, has begun to
bud off two small vesicles in front ; these are the vesicles
of the cerebral hemispheres, which subsequently become the
most conspicuous part of the brain, but up to the end of the
day are still very small and inconspicuous.
16. The notochord, whose origin was described in the
account of the first day (Chap. III. 5), is during the whole of
the second day a very conspicuous object. It is seen as a trans-
parent rod, somewhat elliptical in section (see Fig. 20, ch} 9
lying immediately underneath the medullary canal for the
greater part of its length, and reaching forward in front as
far as below the centre of the second cerebral vesicle, where
it ends either in a point (Remak), or in a rounded knob
(Baer, Dursy, Entwickelungsgeschichte des Kopfes). The ex-
act relations of its termination will be discussed later on.
Round the anterior termination of the notochord, the
medullary canal, which up to the present time has remained
perfectly straight, towards the end of the day begins to curve.
The front portion of the canal, i*e. the fore-brain with its
optic and cerebral vesicles, becomes slightly bent downwards,
so as to form a rounded obtuse angle with the rest of the
embryo. This is the commencement of the so-called cranial
flexure.
17. Lastly, as far as the head is concerned, the rudiment
of the ear appears about this time on the dorsal surface as
a small depression or pitting of the epiblast on either side of
the hind -brain (Fig. 21, au. p).
18. We left the heart as a fusiform body slightly bent to
the right, attached to the under wall of the foregut by the
IV.] THE CIRCULATION. 79
aorta and by its venous end, but with its intermediate portion
quite free. The curvature now increases so much that the
heart becomes almost m -shaped, the venous portion being
drawn up towards the head so as to lie somewhat above (dorsal
to) and behind the arterial portion. (It would perhaps be more
correct to pay that the free intermediate portion is by its
own growth bent downwards, backwards, and somewhat to
the right, while the venous root of the heart is at the same
time continually being lengthened by the carrying back of
that " point of divergence " of the splanchnopleure folds which
marks the union of the omphalo-mesaraic veins into a single
venous trunk). The heart then has at this time two bends,
the one, the venous bend, the right-hand curve of the CQ ;
the other, the arterial bend, the left-hand curve of the ^ .
The venous bend which, as we have said, is placed alsove
arid somewhat behind the arterial bend, becomes marked by
two bulgings, one on either side. These are the rudiments
of the auricles, or rather of the auricular appendages. The
ascending limb of the arterial bend soon becomes conspicuous
as the bulbus arteriosus, while the rounded point of the
bend itself will hereafter grow into the ventricles.
19. The blood-vessels, whose origin during the first half
of this day has been already described, become during the
latter part of the day so connected as to form a complete
system, through which a definite circulation of the blood is now
for the first time (consequently some little while after the
commencement of the heart's pulsation) carried on.
The two primitive aortce have already been described as
encircling the foregut, and then passing along the body of
the embryo immediately beneath the protovertebrse on either
side of the notochord. They are shewn in Fig. 20 a.o in section
as two large rounded spaces lined with spindle-shaped cells.
At first they run as- two distinct canals along the whole
length of the embryo ; but, after a short time, unite at some
little distance behind the head into a single trunk, which
lies in the middle line of the body immediately below the
notochord (Fig. 39). Lower down, nearer the tail, this
single primitive trunk again divides into two aortse, which,
getting smaller and smaller, are finally lost in the small
blood-vessels of the tail. At this epoch, therefore, there are
two aortic arches springing from the bulbus arteriosus, and
80 THE SECOND DAY. [CHAP.
uniting above the alimentary canal in the back of the
to form the single dorsal aorta, which travelling backwards in
the median line divides near the tail into two main branches.
From each of the two primitive aortae, or from each of the
two branches into which the single aorta divides, there is
given off on either side a large branch. These have been
already spoken of as the omphalo-mesaraic arteries. At
this stage they are so large that by far the greater part of
the blood passing down the aorta finds its way into them,
and a small remnant only pursues a straight course into the
continuations of the. aorta towards the tail
Each omphalo-mesaraic artery leaving the aorta at nearly
right angles (at a point some little way behind the backward
limit of the splanchnopleure fold which is forming the ali-
mentary canal), runs outwards beneath the protovertebrse in
the lower range of the mesoblast, close to the hypoblast.
Consequently, when in its course outwards it reaches the
point where the mesoblast is cleft to form the somatopleure
and splanchnopleure, it attaches itself to the latter. Travel-
ling along this, and dividing rapidly into branches, it reaches
the vascular area in whose network of small vessels (and also
to a certain extent in the similar small vessels of the pellucid
area) it finally loses itself.
The terminations of the omphalo-mesaraic arteries in the
vascular and pellucid areas are further connected with the
heart in two different ways. From the network of capillaries,
as we may call them, a number of veins take their origin,
and finally unite into two main trunks, the omphalo-mesaraic
veins. These have already been described as running along the
folds of the splanchnopleure to form the venous roots of the
heart. Their course is consequently more or less parallel to
that of the omphalo-mesaraic arteries, but at some little dis-
tance nearer the head, inasmuch as the arteries run in that
part of the splanchnopleure which has not yet been folded in
to form the alimentary canal. Besides forming the direct
roots of the omphalo-mesaraic veins, the terminations of
the omphalo-mesaraic arteries in the vascular area are also
connected with the sinus terminalis spoken of above as run-
ning almost completely round, and forming the outer margin
of the vascular area. This (Fig. 23, s.v), may be best de-
scribed as composed of two semicircular canals, which nearly
IV.] THE AORTIC ARCHES. 81
meet at points opposite the head and opposite the tail,
thus all but encircling the vascular area between them.
At the point opposite the head the end of each semi-
circle is connected with vessels (Fig. 23), which run straight
in towards the heart along the fold of the splanchnopleure,
and join the right and left omphalo-mesaraic veins. At
the point opposite the tail there is at this stage no such
definite connection. At the two sides, midway between
their head and tail ends, the two semicircles are espe-
cially connected with the omphalo-mesaraic arteries.
The circulation of the blood then during the latter half
of the second day may be described as follows. The blood
brought by the omphalo-mesaraic veins falls into the twisted
cavity of the heart, and is driven thence through the bulbus
arteriosus and aortic arches into the aortic trunk. From the
aorta, by far the greater part of the blood flows into the
omphalo-mesaraic arteries, only a small remnant passing on
into the caudal terminations. From the capillary net-work
of the vascular and pellucid area into which the omphalo-
mesaraic arteries discharge their contents, part of the blood
is gathered up at once into the lateral or direct trunks
of the omphalo-mesaraic veins. Part however goes into
the middle region of each lateral half of the sinus termi-
nalis, and there divides on each side into two streams. One
stream, and that the larger one, flows in a forward direction
until it reaches the point opposite the -head, thence it
returns by the veins spoken of above, straight to the
omphalo-mesaraic trunks. The other stream flows backward,
and becomes lost at the point opposite to the tail. This is
the condition of things during the second day ; it becomes
considerably changed on the succeeding day.
At the time that the heart first begins to beat the
capillary system of the vascular and pellucid areas is not yet
completed; and the fluid which is at first driven by the heart
contains, according to most observers, very few corpuscles.
20. At the close of the second day the single, pair of
aortic arches into which the bulbus arteriosus divides is
found to be accompanied by a second pair, formed in the
same way as the first, and occupying a position a little
behind it. Sometimes even a third pair is added. Of these
aortic arches we shall have to speak more fully later on.
E. 6
82 THE SECOND DAY\ [CHAP.
21. At the latter end of this day, the ridge which we
have already spoken of as the rudiment of the Wolffian duct,
has become distinctly hollow, is in fact no longer a ridge but
a canal. Sections now shew not an irregular group of ordi-
nary mesoblastic cells, but a small cavity surrounded by a
wall of colls ; and these cells are beginning to put on a
columnar character, and thus appear to radiate from the cen-
tral cavity. The canal or duct so formed, the anterior
termination of which is closed, and the posterior not as
yet completely formed, reaches from about the fifth pair
of protovertebrse backwards towards the hind end of the
embryo. The conversion of the ridge into a canal appears
to take place by the cells acquiring a radiating arrange-
ment, and a small hole appearing at the centre where the
points of the cells meet ; this rapidly grows larger till it
reaches the final size of the cavity of the duct,
The exact mode of development of the Wolffian duct is still a matter of some
doubt, although its origin has been investigated by numerous embryologists.
Remak, and after him Kolliker, described it as taking its origin from the
mesoblast of the somatopleure, and appearing about the middle of the second
day, at the external border of the protovertebrae immediately under the epi-
blast, in the form of a solid cord which subsequently became hollow.
liursy (Zeitsch. f. Rat. Med. 1865) gave a very similar account, except that
he regarded it as being derived from the substance of the proto vertebrae,
instead from the somatopleure.
Hensen (Archiv. Microscop. Anat. Bd. in. 1867), an ^ f r 8ome time His,
believed that the duct took origin as a longitudinal involution of the epillast
between the protovertebrse and the lateral mesoblast, in the form of a groove,
which subsequently became closed in and detached from the superficial epiblast,
in a manner very similar to the way in which the lens is formed.
Subsequently His took up the view that it was a product of the proto-
vertebrae, the central cells of these bodies, according to him, protruding as a
ridge along their upper and external angles. He states that at first a distinct
connection ia visible between the Wolffian duct and the central cells of the
protovertebrse.
Waldeyer (Eierstock u. Ei, 1870) has given a totally different account.
Between the external border of the protovertebres, and the point where the
mesoblast splits into somatopleure and splaiichnopleure, there lies a mass of
cells, which we shall have occasion to speak of hereafter as the intermediate cell-
mass. According to Waldeyer, the upper surface of this mass grows up into
a narrow ridge, seen in sections as a tongue-shaped process projecting into the
vacant space (i. e. the space filled with fluid only) which exists below the
epiblast at this point, Later on, this tongue-shaped process is seen to curve out-
wards, and thus to become hook-shaped ; and the point of the hook subsequently
unites with a similar smaller process derived from the more external portions
of the same cell-mass. The small cavity thus seen to be enclosed by a larger
and smaller process, is of course the sectional view of a canal enclosed by a
larger and smaller lidge. This canal is the Wolffian duct. Waldeyer further
believes that the cells which thus form the walls of the duct are primarily epi-
IV.] THE AMNION. 83
blastic in origin, having been separated from the epiblast at the epoch of the
apparent fusion of the epiblast and mesoblast in the region of the primitive
streak or axis-cord of His This view, prompted as it evidently is by theoretical
considerations, must be regarded as untenable, since the primitive streak has
nothing whatever to do with the permanent body of the embryo.
Quite recently Romiti (Archiv. Microscop. Anat. x. 1874) has described the
Wolffian duct as being formed by an involution of the epithelium of die pleuro-
peritoneal cavity, in the form of a longitudinal groove which is thrust up into
the superior portions of the intermediate cell-mass, and the communication of
which with the pleuroperitoneal cavity is speedily obliterated. Such a mode
of origin recommends itself to the embryologtetj inasmuch as it is certainly the
way in which, as we shall see, the Wolffian duct is formed in Amphibia and
Osseous Fishes. For that very reason however it should be received with
caution; all the more since the sections drawn by Romiti, and described as
supporting his views, evidently belong to a stage considerably later than that
at which the duct first distinctly appears. We hope to be able to shew, in the
second part of this work, that the mode of development of the Wolffian duct
described above, and which we believe to be the real one> is not so abnormal as
it might at first sight be supposed to be.
22. The amnion, especially the anterior or head-fold,
advances in growth very rapidly during the second day, and
at its close completely covers the head and neck of the
embryo ; so much so that it is necessary to tear or remove it
when the head has to be examined in hardened opaque speci-
mens. The tail and lateral folds of the amnion, though still
progressing, lag considerably behind the head-fold.
23. The chief events then which occur during the
second half of the second day are as follows ;
1. The second and third cerebral vesicles make their
appearance behind the first.
2. The optic vesicles spring as hollow buds from the
lateral, and the vesicles of the cerebral hemispheres from the
front portions of the first cerebral vesicle.
3. The first rudiment of the ear is formed as an involu-
tion of the epiblast on the side of the hind-brain or third
cerebral vesicle.
4. The first indications of the cranial flexure become
visible.
5. The head-fold, and especially the splanchnopleure
moiety, advances rapidly backwards ; the head of the embryo
is in consequence more definitely formed, The tail-fold also
becomes distinct.
6. The curvature of the heart increases ; the first rudi-
ments of the auricles appear.
7. The circulation of the yolk-sac is completed.
8. The amnion grows rapidly.
CHAPTER V.
THE CHANGES WHICH TAKE PLACE DURING THE THIRD DAY.
1. OF all days in the history of the chick within the egg
this perhaps is the most eventful ; the rudiments of so many
important organs now first make their appearance.
On opening an egg on the third day, the first thing which
attracts notice is the diminution of the white of the egg.
This seems to be one of the consequences of the functional
activity of the newly-established vascular area whose blood-
vessels are engaged either in directly absorbing the white or,
as is more probable, in absorbing the yolk, which is in turn
replenished at the expense of the white. The absorption,
once begun, goes on so actively that, by the end of the day,
the decrease of the white is very striking.
2. The blastoderm has now spread over about half the
yolk, the extreme margin of the opaque area reaching about
half-way towards the pole of the yolk opposite to the embryo.
The vascular area, though still increasing, is much smaller
than the total opaque area, being in average-sized eggs about
as large as a florin. Still smaller than the vascular area is the
pellucid area in the centre of which lies the rapidly growing
embryo.
3. During the third day the vascular area is not only a
means for providing the embryo with nourishment from the
yolk, but also, inasmuch as by the diminution of the white it
is brought close under the shell and therefore fully exposed
to the influence of the atmosphere, serves as the chief organ
of respiration.
This in fact is the period at which the vascular area may
be said to be in the stage of its most complete development ;
CH. V.]
THE VASCULAR AREA.
85
for though it will afterwards become larger it will at the
same time become less definite and less important. We
may therefore, before we proceed, add a few words to the
description of it given in the last chapter.
L.Of
DIAGRAM OP THE CIRCULATION OF THE YOLK-SACK AT THE END OF THE
THIRD DAY OF INCUBATION.
H. heart. A A. the second, third and fourth aortic arches; the first
has become obliterated in its median portion, but is continued at its
proximal end as the external carotid, and at its distal end as the internal
carotid. AO. dorsal aorta. L. Of. A. left omphalo-mesaraic artery.
R. Of. A. right omphalo-mesaraic artery. S. T. sinus terminals. L. Of. left
omphalo-mesaraio vein. R. Of. right omphalo-mesaraic vein. S. V. sinus
venosus. D. C. ductus Cuvieri. S. Ca. V. superior cardinal vein.
V.Ca. inferior cardinal vein. The veins are marked in outline and the arteries
are made black. The whole blastoderm has been removed from the egg
and is supposed to be viewed from below. Hence the left is seen on the
right, and vice versa.
86 THE THIKD DAY. [CHAP.
The blood leaving the body of the embryo by the omphalo-
mesaraic arteries (Fig. 23, R. Of.A., L. Of. A.), is carried to
the small vessels and capillaries of the vascular area, a small
portion only being appropriated by the pellucid area.
From the vascular area part of the blood returns directly
to the heart by the main lateral trunks of the omphalo-
mesaraic veins, R. Of., L. Of. During the second day these
venous trunks joined the body of the embryo considerably
in front of, that is, nearer the head than, the corresponding
arterial ones. Towards the end of the third day, owing to the
continued lengthening of the heart, the veins and arteries
run not only parallel to each other, but almost in the same
line, the points at which they respectively join and leave
the body being nearly at the same distance from the head.
According to Von Baer and other observers the veins in the
vascular area are placed nearer the surface thau are the
arteries. Close to the body the reverse is the case ; near the
body therefore they cross over each other.
The rest of the blood brought by the omphalo-mesaraic
arteries finds its way into the lateral portions of the sinus
terminalis, S.T., and there divides on each side into two
streams. Of these, the two which, one on either side, flow
backward, meet at a point about opposite to the tail of the
embryo, and are conveyed along a distinct vein which, run-
ning straight forward parallel to the axis of the embryo,
empties itself into the left omphalo-mesaraic vein. The
two forward streams reaching the gap in the front part of
the sinus terminalis fall into either one, or in some cases
two veins, which run straight backward parallel to the axis
of the embryo, and so reach the roots of the heart. When
one such vein only is present it joins the left omphalo-
mesaraic trunk ; where there are two they join the left and
right omphalo-mesaraic trunks respectively. The left vein is
always considerably larger than the right; and the latter
when present rapidly gets smaller and speedily disappears.
The chief differences then between the peripheral cir-
culation of the second and of the third day are due to the
greater prominence of the sinus terminalis and the more
complete arrangements for returning the blood from it to the
heart. After this day, although the vascular area will go on
increasing in size until it finally all but encompasses the
V.] CHANGE OF POSITION. 87
yolk, the prominence of the sinus terminalis will become less
and less in proportion as the respiratory work of the vascular
area is shifted on to the allantois, and its activities confined
to absorbing nutritive matter from the yolk.
4. The folding in of the embryo makes great progress
during this day. Both head and tail have become most
distinct, and the side folds which are to constitute the
lateral walls have advanced so rapidly that the embryo is
now a bona fide tubular sac, connected with the rest of
the yolk by a broad stalk. This stalk, as was explained in
Chap, n, is double, and consists of an inner splanchnic stalk
continuous with the alimentary canal, which is now a tube
closed at both ends and open to the stalk along its middle
third only, and an outer somatic stalk continuous with the
body- walls of the embryo, which have not closed nearly to
the same extent as the walls of the alimentary canal. (Com-
pare Fig. 8. A and B, which may be taken as diagrammatic
representations of longitudinal and transverse sections of an
embryo of this period.)
5. The embryo is almost completely covered by the
amnion. Before the close of the day the several amniotic
folds will have met along a line over the back of the embryo.
Their complete coalescence, and the obliteration of their line
of junction, will however not take place till the following
day.
6. During this day a most rema.rkable change takes
place in the position of the embryo. Up to this time it has
been lying symmetrically upon the yolk with the part
which will be its mouth directed straight downwards. It
now turns round so as to lie on its left side.
This important change of position is almost invariably
completed on the third day. At the same time the left
omphalo-mesaraic vein, the one on the side on which the
embryo comes to lie, grows very much larger than the
right, which henceforward gradually dwindles and finally dis-
appears.
7. Coincidently with the change of position the whole
embryo begins to be curved on itself. This curvature of the
body, Fig. 46, becomes still more marked on the fourth day.
8. In the head very important changes take place.
One of these is the cranial flexure. Figs. 24, 25. This (which
THE THIRD DAY.
[CHAP;
must not be confounded with the curvature of the body just
referred to) we have already seen was commenced in the
course of the second day, by the bending downwards of the
head round a point which may be considered as the extreme
end either of the notochord or of the alimentary canal.
CHICK OF THE THIRD DAY (54 HOURS) VIEWED FROM UNDERNEATH AS A
TRANSPARENT OBJECT.
a', the outer amniotic fold or false amnion. This is very conspicuous around
the head, hut may also be seen at the tail.
a. the true anmion, very closely enveloping the head, and here seen only between
the projections of the several cerebral vesicles. It may also be traced at
the tail, t.
In the embryo of which this is a drawing the head-fold of the amnion
reached a little farther backward than the reference u, but its limit could not be
V.] THE CRANIAL FLEXURE. 89
distinctly seen through the body of the embryo. The prominence of the false
atnnion at the head is apt to puzzle the student ; but if he bears in mind the
fact, which could not well be shewn in Fig. 8, that the whole arnniotic fold, both
the true and the false limb, is tucked in underneath the head, the matter will
on reflection become intelligible.
C. H. cerebral hemisphere. F. B. fore-brain or vesicle of the third ventricle.
M '. B. mid-brain. H. B. hind-brain. Op. optic vesicle. Ot. otic vesicle.
OfV. omphalo-mesaraic veins forming the venous roots of the heart. The trunk
on the right hand (left trunk when the embryo is viewed in its natural
position from above) receives a large branch, shewn by dotted lines, coming
from the anterior portion of the sinus terminalis Ht. the heart, now com-
pletely twisted on itself. Ao. the bulbus arteriosus, the three aortic arches
being dimly seen stretching from it across the throat, and uniting into the
aorta, sti.l more dimly seen as a curved dark line running along the body.
The other curved dark line by its side, ending near the reference y, is the
notochord ch.
About opposite the line of reference x the aorta divides into two trunks, which
running in the line of the somewhat opaque proto vertebrae on either side,
are not clearly seen. Their branches however, Ofa t the omphalo-mes'araic
arteries, are conspicuous and are seen to curve round the commencing side
folds.
Pv. protovertebrae. Below the level of the omphalo-mesaraic arteries the vertebral
plates are but imperfectly cut up into proto vertebrae, and lower down still,
not at all.
x is placed at the "point of divergence" of the splanchnopleure folds. The
blind foregut begins here and extends about up to y, the more transparent
space marked by that letter being partly due to the presence there of the
cavity of the alimentary canal, x therefore marks the present hind limit of
the splanchnopleure folds. The limit of the more transparent somato-
pleure folds cannot be seen.
It will be of course understood that all the body of the embryo above the
level of the reference x, is seen through the portion of the yolk-sac (vascular
and pellucid area), which has been removed with the embryo from the egg, as
well as through the double amniotic fold.
We may repeat that, the view being from below, whatever is described in
the natural position as being to the right here appears to be left, and vice
versa.
The flexure progresses rapidly, the front-brain being
more and more folded down till, at the end of the third day,
it is -no longer the first vesicle or fore-brain, "but the second
cerebral vesicle or mid-brain, which occupies the extreme
front of the long axis of the embryo. In fact a straight
line through the long axis of the embryo would now pass
through the mid-brain instead of, as at the beginning of the
second day, through the fore-brain, so completely has the
front end of the neural canal been folded over the end of
the notochord. The commencement of this cranial flexure
gives the body of an embryo of the third day somewhat the
appearance of a retort, the head of the embryo corresponding
00
THE THIRD DAY.
[CHAP.
to the bulb. On the fourth day the flexure is still greater
than on the third, but on the fifth and succeeding days it
becomes less obvious owing to the filling up of the parts of the
skull.
FIG. -25.
HEAD OP A CHICK OF THE THIRD DAY VIEWED SIDEWAYS AS A TRANSPARENT
OBJECT. (From Huxley.)
I. a. the vesicle of the cerebral hemisphere. I. 6. the vesicle of the third
ventricle (the original fore-brain) ; at its summit is seen the projection of
the pineal gland e.
Below this portion of the brain is seen, in optical section, the optic vesicle ct
already involuted with its thick inner and thinner outer wall (the letter a is
placed on the junction of the two, the primary cavity being almost obliterated).
In the centre of the vesicle lies the lens, the shaded portion (represented too
large), being the expression of its cavity. Below the lens between the two limbs
of the horse-shoe is the choroidal fissure.
II. the mid-brain, now, owing to the cranial flexure, opposite the end of
the alimentary canal. III. the hind-brain. V. the rudiments of the fifth
cranial nerve, VII. of the seventh. Below the seventh nerve is seen the
auditory vesicle b. The head having been subjected to pressure, the vesicle
appears somewhat distorted as if squeezed out of place. The orifice is not yet
quite closed up.
i. the inferior maxillary process of the first visceral fold. Below, and to
the right of this, is seen the first visceral cleft, below that again the second
visceral fold (2), and lower down the third (3) and fourth (4) visceral folds. In
front of the folds (i.e. to the left) is seen the arterial end of the heart, the
aortic arches being buried in their respective visceral folds.
f. represents at its lowest part the cavity of the alimentary canal as seen
through the transparent body of the embryo; at the upper part below the brain
it is difficult to distinguish between the transparency due to the presence of the
cavity of the alimentary canal, and that caused by the character of the meso-
blast at the base of the skull, which, being formed of stellate cells with largely
developed clear spaces or vacuoles, allows the light to pass readily through it.
Near its upper end below the mid-brain is seen a small conical process, the
rudiment of the infundibulum.
9. The two vesicles of the cerebral hemispheres which
V.] THE FORE-BRAIX. 91
on the second day began to grow out from the front of the
fore-brain, increase rapidly in size during the third day, so
much so that by the end of the day each of them (Fig. 24, C7/,
Fig. 25, la) is as large or larger than the original fore-brain
from which they both sprang, and they form together a most
conspicuous part of the brain. In their growth they pash
aside, the optic vesicles, and thus contribute largely to the
roundness which the head is now acquiring. Each vesicle
possesses a cavity, known afterwards as a lateral ventricle ,
which, though quite separate from its fellow, is continuous
with the cavity of the fore-brain.
Owing to the development of these cerebral hemispheres,
the original fore-brain no longer occupies the front position
(Fig. 24, FB, Fig. 25, 16), and ceases to be the conspicuous
object that it was. Inasmuch as its walls will hereafter be de-
veloped into the parts surrounding the so-called third ventricle
of the brain, we shall henceforward speak of it as the vesicle
of the third ventricle, or, inasmuch as it soon comes to lie
between the expanded posterior ends of the cerebral hemi-
spheres, as the 'tween brain.
On the summit of the 'tween brain there may now be
seen a small conical projection, the rudiment of the pineal
gland' (Fig. 25, e\ while the centre of the floor is produced
into a funnel-shaped process, the infundibulum (Fig. 22, d\
which, stretching towards the extreme end of the alimentary
canal, joins the pituitary body.
The development of the pituitary body or hypophysis cerebri has been the
subject of considerable controversy amongst embryologists. Von Baer (loc. cit.)
and Smidt (Zeitschrift fur Wiss. Zoologie, 1862, JB. XT, p. 43) believed that the
base of the fore-brain, or vesicle of the third ventricle, became produced into a
downward process, the 'infundibulum,' which subsequently became expanded at
its termination to form the pituitary body.
Kathke (Archiv fiir Anatomic und Physiologic 1838, Bd. v.) states that very
early a diverticulum is produced from the upper end of the alimentary canal, which
passes backwards and meets the process of the brain called the infundibulum.
This diverticulum subsequently loses all connection with the epithelium of the
: digestive canal, and, uniting with the infundibulum, forms the pituitary body.
Dursy (Entwicklungsgeschichte des Kopfes, Tubingen, 1869) states that both
ithe end of the notochord and the epithelium of the alimentary canal take, part
j in the formation of the pituitary body. The apparent diverticulum of the ali-
1 mentary canal is not so much a true diverticulum, as a part of the alimentary
, can;il constricted off from the remainder by the cranial flexure.
Keichert (Entivicklungsleben im Wirbelthierreich. Berlin, 1840) states that
the pituitary body is formed from the remains of the front end of notochord.
92 THE THIRD DAY. [CHAP.
Subsequently however (Der Ban des menschlichen Gehirns) he supposed that
it was formed from the pia mater.
Rathke also subsequently (EntwicTclunsgescJiiclite der Wirbelthiere, Leipzig,
1861) gave up his former view, and believed that the diverticulum of the
alimentary canal disappeared, but that the pituitary body was formed from
the mesoblast in front of the clinoid process.
Wilhelm Miiller (Ueber die Entwicklung und Ban der Hypophysis und des
Processes infundibuli cerebri. Jenaiscke Zeitschrift, Bd. vr. 1871) has recently
written an elaborate memoir on the development and anatomy of the pituitary
body and infundibulum in all the orders of Vertebrates, of which the following
is an abstract.
In order to understand the formation of the diverticulum from the ali-
mentary canal which forms the pituitary body, we must remember that at
first the hypoblast of the throat closely underlies the notochord, and beyond
the end of the notochord is almost in contact with the base of the vesicle of the
third ventricle. When the cranial flexure occurs, which it will be remembered
takes place about an axis coinciding with the end of the notochord, the
hypoblast, which closely underlies the base of the brain, becomes at the same
time bent; and as the angle of the flexure becomes an acute angle, a wedge-
shaped space lined by hypoblast is as it were constricted off from the alimen-
tary canal. In this way there is formed a diverticulum of hypoblast which
passes forwards from the alimentary canal to the base of the fore-brain
a diverticulum not produced by a forward growth from the alimentary canal,
but solely due to the cranial flexure constricting off a wedge-shaped portion
of the alimentary canal. This we may call the pituitary diverdculum. When
the cranial flexure commences the end of the notochord becomes bent down-
ward, and, ending in a somewhat enlarged extremity, comes in contact with the
termination of the pituitary diverticulum. The mesoblast around and at the
front of the end of the notochord increases and grows up, in front of the
notochord and behind the vesicle of the third ventricle, to form the posterior
clinoid process. The base of the vesicle of the third ventricle at the same
time grows downwards towards the pituitary diverticulum and forms what is
known as the infundibulum. This state of things may be observed on the
third day. On the fourth day the mesoblast tissue around the notochord
increases in quantity, and the end of the notochord, though still bent down-
wards, recedes a little from the termination of the pituitary diverticulum, which
is still a triangular space with a wide opening into the alimentary canal.
On the fifth day, the opening of the pituitary diverticulum into the
alimentary canal lias become narrowed* and around the whole diverticulum a
formation of mesoblast-cells lias commenced. Behind it the clinoid process
has become cartilaginous, while to the side and in front it is enclosed by the
trabeculse. At this stage, in fact, we have a diverticulum from the alimentary
canal passing through the base of skull to the infundibulum. The end of the
notochord has at this stage become atrophied, so that it is separated by a
considerable interval from the pituitary body.
On the seventh day the mesoblast around the pituitary diverticulum has
grown into a complete investment of spindle-shnped cells, and the communi-
cation between the cavity of the diverticulum and that of the throat has become
still narrower. The diverticulum is all but converted into a vesicle, and its hypo-
blast walls have commenced to send out into the mesoblastic investment solid
processes, which form the first commencement of the true pituitary body. The
infundibulum now appears as a narrow- process from the base of the vesicle of
the third ventricle, which approaches, but does not unite with the pituitary vesicle.
This latter lies in the space between the basi- and the presphenoid, and is
V.] THE MID-BRAIN AND HIND-BRAIN. 93
completely surrounded by a ring of cartilage. The mesoblast-cells immediately
around it do not. however, exhibit any signs of becoming cartilaginous.
By the tenth day the opening of the pituitary vesicle into the threat becomes
almost obliterated, and the lumen of the vesicle itself very much diminished.
The body itself consists of anastomosing cords of hyp 'blast-cells, the meso-
blast between which has already commenced to become vascular. The cords
or masses of hypoblast cells are surrounded by a delicate membrana propria,
and a few of them possess a small lumen. The infundibulum has increased in
length.
On the twelfth day the communication between the pituitary vesicle and
the throat is entirely obliterated, but a solid cord of cells still connects the
two. The vessels of the pia mater of the vesicle of the third ventricle have
become connected with the pituitary body, and the infundibulum has grown
down along its posterior border.
In the later stages, all connection is lost between the pituitary body and
the throat, and the former becomes connected with the elongated processes
infundibuli.
Such is Wilhelm Miiller's account* Goette , however (Archiv. Micr. Anat.
IX. p. 397), has recently given reasons for thinking that the pituitary diverti-
culum arises not from the closed foregut, lined \\ith hypoblast, but from the
buccttl cavity lined with epiblast. He states that in its earlier stages it may be
seen to start on the oral side of the partition, which for some time divides the
secondarily formed buccal cavity from the primarily formed foregut, and
therefore, belonging to the former, cannot be regarded as the natural anterior
termination of the latter.
Beyond an increase in size, which it shares with nearly
all parts of the embryo, and the change of position to which
we have already referred, the mid-brain undergoes no great
alteration during the third day. Its roof will ultimately
become developed into the corpora bigemina or optic lobes
(quadrigemina in mammals), its floor will form the crura
cerebri, and its cavity will be reduced to the narrow canal
known as the iter a tertio ad quartum ventriculum.
In the hind-brain, or third cerebral vesicle, that part
which lies nearest to the mid-brain, becomes during the
third day marked off from the rest by a slight constric-
tion. This distinction, which becomes much more evident-
later on by a thickening of the walls and roof of the front
portion, separates the hind-brain into the cerebellum in front,
and the medulla oblongata behind. While the walls of the
cerebellar portion of the hind -brain become very much
thickened as well at the roof as at the floor and sides, the
roof of the posterior or medulla oblongata portion thins out
into a mere membrane, forming a delicate covering to the
cavity of the vesicle (Fig. 26. iv), which here becoming
broad and shallow with greatly thickened floor and sides, is
94 THE THIRD DAY. [CHAP.
known as the fourth ventricle, subsequently overhung by the
largely developed posterior portion of the cerebellum.
The third day, therefore, marks the distinct differentiation
of the brain into its fundamental parts: the cerebral hemi-
spheres, the central masses round the third ventricle, the
corpora bigemina, the cerebellum and the medulla oblongata;
the original cavity of the neural canal at the same time,
passing from its temporary division of three single cavities
into the permanent arrangement of a series of connected
ventricles, viz. the lateral ventricles, the third ventricle, the
iter (with a prolongation into the optic lobe on each side),
and the fourth ventricle.
10. At the same time that the outward external shape
of the brain is thus being moulded, internal changes are
taking place in the whole neural canal. These are best seen
in sections.
At its first formation, the section of the cavity of the
neural canal is round or nearly so.
About this time, however, the lining of involuted epiblast
along the length of the whole spinal cord becomes very much
thickened at either side, while increasing but little at the
mid-points above and below. The result of this is that the
cavity as seen in section (Fig. 44), instead of being circular,
has become a narrow vertical slit, almost completely filled in
on either side.
In the region of the brain the thickening of the lining
epiblast follows a somewhat different course. While almost
everywhere the sides and floor of the canal are greatly thick-
ened, the roof in the region of the various ventricles, not of
the fourth only, but of the others as well, becomes excessively
thin, so as to form a membrane reduced to almost a single
layer of cells. (Fig. 26. IV.)
11. In the preceding chapter we saw how the first cere-
bral vesicle, by means of lateral outgrowths followed by
constrictions, gave rise to the optic vesicles. These and the
parts surrounding them undergo on the third day changes
which result in the formation of the eyeball.
At their first appearance the optic vesicles stand out at
nearly right angles to the long axis of the embryo (Fig. 15),
and the stalks which connect them with the fore-bram are'
short and wide. We have already said (p. 77) that the con-
THE OPTIC VESICLES.
95
strictions. which give rise to the stalks take place chiefly from
above downwards, and also somewhat inwards and backwards.
Thus from the first the vesicles appear to spring from the
under part of the fore-brain.
FIG. 26.
cc
jiOA
SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF THE THIRD
DAY OF INCUBATION.
IV Fourth ventricle. The section shews the very thin roof and thicker sides
of the ventricle.
Ch. Notochord (diagrammatic shading).
CV. Anterior cardinal vein.
CC. Involuted auditory vesicle. CC points to the end which will form the
cochlear canal. RL. Kecessus labyrinth!, liy. hypoblast lining the alimen-
tary canal, hy is itself placed in the cavity of the alimentary canal, in that
part of the canal which will become the throat. The lower (anterior) wall of
the canal is not shewn in the section, but on each side are seen portions of a pair
of visceral arches. In each arch is seen the section of the aortic arch A OA
belonging to the visceral arch. The vessel thus cut through is running upwards
towards the head, being about to join the dorsal aorta AO. Had the section
been nearer the head, and carried through the plane at Which the aortic arch
curves round the alimentary canal to reach the mesoblast above it, A OA and A
would have formed one continuous curved space. In sections lower down in
the back the two aorta, A 0, one on either side would be found fused into one
median canal.
The shading of the mesoblast is diagrammatic ; it is here a uniform mass
of spindle-shaped cells ; there being in this region no differentiation into proto-
vertebre.
96 THE THIRD DAY. [dlAP,
As the vesicles of the cerebral hemispheres grow out
rapidly from the front and under portions of the first cerebral
vesicle, they seem to thrust the optic vesicles apart and to
the sides.
Thus these, instead of standing out from the extreme
front, come to be placed at the sides of the head, the stalks,
which are correspondingly lengthened and narrowed, running
obliquely downwards and inwards from the vesicles to open
into the cavity of the brain at its base. Their openings are
at first placed close to each other at the junction of the
cerebral hemispheres with the remnant of the fore-brain (now
called the vesicle of the third ventricle), so that the cavities
of the two optic vesicles may be said to communicate directly
both with each other and with the cavities of the cerebral
hemispheres. The later connection is however soon lost, and
the stalks of the optic vesicles then open exclusively into the
third ventricle. At the same time the floor of the third ven-
tricle, during the occurrence of the cranial flexure, grows
down and thrusts apart the openings of the two optic stalks.
At a later date the stalks shift their position backwards, and
thus become connected chiefly with the base of the mid-
brain.
While these changes have been going on in the optic
stalks, development has also proceeded in the region of
the vesicles themselves, and given rise to the rudiments of
the retina, lens, vitreous humour, and other parts of the eye.
The changes through which these are formed are of a
somewhat complicated character, and not a few points in
reference to them are still involved in some doubt.
Towards the end of the second day, the external or super-
ficial epiblast which covers, and is in all but immediate
contact with the most projecting portion of the optic vesicle,
becomes thickened. This thickened portion is then driven
inwards in the form of a shallow open pit with thick walls
(Fig. 27 A. x), carrying before it the front wall (r) of the optic
vesicle. To such an extent does this involution of the super-
ficial epiblast take place, that the front wall of the optic
vesicle is pushed close up to the hind wall, and the cavity of
the vesicle becomes almost obliterated (Fig. 27, ).
The bulb of the optic vesicle is thus converted into a cup
with double walls, containing in its cavity the portion of
V.] THE OPTIC CUP. 97
involuted epiblast. This cup, in order to distinguish its
cavity from that of the original optic vesicle, is generally
called the secondary optic vesicle. We may, for the sake of
brevity, speak of it as the optic cup ; in reality it never is a
vesicle, since it always remains widely open in front. Of its
double walls the inner or anterior (Fig. 27 B, r) is formed
from the front portion, the outer or posterior (Fig. 27
B, u) from the hind portion of the wall of the primary optic
vesicle. The inner or anterior (r), which very speedily
becomes thicker than the other, is converted into the retina ;
in the outer or posterior (u), which remains thin, pigment is
eventually deposited, and it ultimately becomes the tesselated
pigment-layer of the choroid.
FJG. 27.
A. B.
DIAGRAMMATIC SECTIONS ILLUSTRATING THE FORMATION OP THE EYE.
(After Eemak.)
In A, the thin superficial epiblast h is seen to be thickened at x, in front of
the optic vesicle, and involuted so as to form a pit o, the mouth of which
has already begun to close in. Owing to this involution, which forms the
rudiment of the lens, the optic vesicle is doubled in, its front portion r being
pushed against the back portion u, and the original cavity of the vesicle thus
reduced in size. The stalk of the vesicle is shewn as still broad.
In B, the optic vesicle is still further doubled in so as to form a cup with a
posterior wall u and an anteiior wall r. In the hollow of this cup lies the lens /,
now completely detached from the superficial epiblast x, h. Its cavity is still
shewn. The cavity of the stalk of the optic vesicle is already much narrowed.
By the closure of its mouth the pit of involuted epiblast
becomes a completely closed sac with thick walls and a small
central cavity. (Fig. 27 B, I). At the same time it breaks
away from the external epiblast, which forms a continuous
layer in front of it, all traces of the original opening being
lost. There is thus left lying in the cup of the secondary
optic vesicle, an isolated elliptical mass of epiblast. This is
E. 7
98 THE THIRD DAY. [CHAP.
the rudiment of the lens. The small cavity within it speedily
becomes still less by the thickening of the walls, especially
of the hinder one.
At its first appearance the lens is in immediate contact
with the anterior wall of the secondary optic vesicle (Fig.
27 .B). In a short time however, the lens is seen to lie in
the' mouth of the cup (Fig. SOD), a space (vh) (which is
subsequently occupied by the vitreous humour) making its
appearance between the lens arid anterior wall of the vesicle.
In order to understand how this space is developed, the
position of the optic vesicle and the relations of its stalk
must be borne in mind.
FIG. 28.
DIAGRAMMATIC SECTION OF THE EYE AND THE OPTIC NERVE AT AN
EARLY STAGE (from Lieberkiihn),
to shew the lens I occupying the whole hollow of the optic cup, the inclination of
the stalk s to the optic cup, and the continuity ol the cavity of the stalk s' with
that of the primary vesicle c ; r, anterior, u posterior wall of the optic cup.
The vesicle lies at the side of the head, and its stalk is
directed downwards, inwards and backwards. The stalk in
fact slants away from the vesicle. Hence when the involu-
tion of the lens takes place, the direction in which the front
wall of the vesicle is pushed in is not in a line with the axis
of the stalk as for simplicity's sake has been represented in
the diagram Fig. 27, but forms an obtuse angle with that
axis, after the manner of Fig. 28, where s represents the
cavity of the stalk leading away from the almost obliterated
cavity of the primary vesicle.
Fig. 28 represents the early stage at which the lens fills
the whole cup of the secondary vesicle. The subsequent
state of affairs is brought about through the growth of the
THE CHOROIDAL FISSURE.
99
walls of the cup taking place more rapidly than that of the
lens, or in other words to the cavity of the cup dilating.
But this growth or this dilatation does not take place equally
in all parts, of the cup. The walls of the cup rise up all
round except that point of the circumference of the cup
which is opposite the middle (from side to side) of the stalk.
While elsewhere the walls increase rapidly in height, carrying
so to speak the lens with them, at this spot, which in the
natural position of the eye is on its under surface, there is
no growth : the wall is here imperfect, and a gap is left.
Through this gap, which afterwards receives the name of the
choroidal fissure, a way is open from the mesoblastic tissue
surrounding the optic vesicle and stalk into the interior of
the cavity of the cup.
DIAGRAMMATIC EEPRESENTATION OF THE EYE OF THE CHICK OF ABOUT THE
THIRD DAY AS SEEN WHEN THE HEAD IS VIEWED FROM UNDERNEATH AS
A TRANSPARENT OBJECT.
/ the lens, I' the cavity of the lens, lying in the hollow of the optic cup.
r the anterior, u the posterior wall of the optic cup, c the cavity of the
primary optic vesicle, now nearly obliterated. By inadvertence u has been
drawn in some places thicker than r, it should have been thinner through-
out.
s the stalk of the optic cup with s' its cavity, at a lower level than the cup
itself and therefore out of focus ; the dotted line indicates the continuity of
the cavity of the stalk with that of the primary vesicle.
The line 2, 2, through which the section shewn in Fig. 30 F is supposed
to be taken, passes through the choroidal fissure.
From the manner of its formation the gap or fissure is
evidently in a line with the axis of the optic stalk, and in
100 THE THIED DAY. [CHAP.
order to be seen must be looked for on the under surface
of the optic vesicle. In this position it is readily recog-
nized in the transparent embryo of the third day, Figs. 25
and 29.
Bearing in mind these relations of the gap to the optic
stalk, the reader will understand how sections of the optic
vesicle at this stage present very different appearances
according to the plane in which the sections are taken.
When the head of the chick is viewed from underneath
as a transparent object the eye presents very much the ap-
pearance represented in the diagram Fig. 29.
D. Diagrammatic section taken perpendicular to the plane of the paper,
along the line y t y, Fig. 29. The stalk is not seen, the section falling
quite out of its region, vh, hollow of optic cup filled with vitreous
humour; other letters as in Fig. -27 B.
E. Section taken parallel to the plane of paper through Fig. 29, so far behind
the front surface of the eye as to shave off a small portion of the posterior
surface of the lens I, but so far in front as not to be carried at all through
the stalk. Letters as before ; /, the choroidal fissure.
F. Section along the line z, z, perpendicular to the plane of the paper, to shew
the choroidal fissure /, and the continuity of the cavity of the optic stalk
with that of the primary optic vesicle. Had this section been taken a
little to one side of the line 2, z, the wall of the optic cup would have
extended up to the lens below as well as above. Letters as before.
A section of such an eye taken along the line y, per-
pendicular to the plane of the paper, would give a figure
corresponding to that of Fig. 30 D. The lens, the cavity
and double walls of the secondary vesicle, the remains of the
primary cavity, would all be represented (the superficial
epiblast of the head would also be shewn) ; but there
would be nothing seen of either the stalk or the fissure.
If on the other hand the section were taken in a plane
parallel to the plane of the paper, at some distance above the
V.] THE - CHOROID^L > FtSSURK.
101
level of the stalk, some such' gg^e:wcitrd' 5 b e''-gai^e3 as that
shewn in Fig. 30 E. Here the fissure / is obvious, and the
communication of the cavity vh of the secondary vesicle with
the outside of the eye evident; the section of course wo aid
not go through the superficial epiblast. Lastly, a section,
taken perpendicular to the plane of the paper along the line
z, i. e. through the fissure itself, would present the ap-
pearances of Fig. 30 F, where the wall of the vesicle is
entirely wanting in the region of the fissure marked by the
position of the letter f.
The fissure such as we have described it exists for a short
time only. Its lips come into contact, and unite (in the
neighbourhood of the lens, directly, but in the neighbourhood
of the stalk, by the intervention of a structure which we
shall describe presently), and thus the cup-like cavity of the
secondary optic vesicle is furnished with a complete wall
all round. The interior of the cavity is filled by the
vitreous humour, a clear fluid in which are a few scat-
tered cells.
In the foregoing account of the formation of the secondary optic vesicle,
and of the fissure, as the results of a process of unequal growth, we have fol-
lowed the account of Lieberkiihn (Uber das Auge des Wirbelthier embryos,
Schriften der Geseilsehaft zur Betbrderung der gesammten Naturwisserischatten
zu Marburg. Bd. 10. 1872). Their origin is more generally described as
being due to a doubling up of the primary vesicle from the side along the
line of the fissure at the same time that the lens is being thrust in in front. In
mammalia, the doubling up is said to involve the optic stalk, which becomes
flattened (whereby its original cavity is obliterated) and then folded in on itself,
so as to embrace a new central cavity continuous with the cavity of the
vitreous humour.
According to Lieberkiihn the optic stalk in birds is never so folded up,
but is converted into the optic nerve by the gradual obliteration of its primary
central cavity through increased thickening of the walls. The optic nerve
of the bird, moreover, contains no arteria centralis retina, while the involu-
tion of the optic stalk into the optic nerve was supposed to have for its purpose
the introduction of a quantity of mesoblast into the interior of nerve, in order
to form the artery.
According to Remak and the majority of observers after him, no mesoblast
whatever exists between the external epiblast and the optic vesicle, at the
point where the former is thrust inwards to form the lens, and hence this
organ carries with it in its involution no mesoblast whatever to serve as a
rudiment of either the vitreous humour or the capsule of the lens. They
described the vitreous humour as being formed entirely out of the meso-
blast which was intruded from the exterior of the eye through the choroidal
fissure, and Kolliker considered the capsule of the lens as a sort of cuticular
excretion from the surface of the lens itself. Lieberkiihn on the other hand
states that shortly after the commencement of the involution of the lens there
102 v< THE T&IRD DAY. [CHAP.
may be^alraady ^V^lt^tli^n^y^r'Qf-me^oblaRt, interposed between it and the
optic vesicle. This layer is carried inward during the involution, and from
it both the vitreous humour and the capsule of the lens take their origin.
Jn birds it is very difficult to be sure of the existence of this layer, though
Lieberkiihn says that in mammals it is conspicuous ; and even if its existence
be admitted, it still remains doubtful whether it gives rise to the whole vi-
treous humour, or to the capsule of the lens only; though the latter view is most
probable.
During the changes in the optic vesicle just described, the
surrounding mesoblast takes on the characters of a distinct
investment, whereby the outline of the eyeball is definitely
formed. The internal portions of this investment, nearest to
the retina, become the choroid (i.e. the chorio-capillaris,
and the lamina fusca, the pigment epithelium, as we have
seen, being derived from the epiblastic optic cup), and pig-
ment is subsequently deposited in it. The remaining external
portion of the investment forms the sclerotic.
The complete differentiation of these two coats of the eye
does not however take place till a late period. Along the
line of the choroidal fissure the pigment is wanting. Con-
sequently in embryos of an age when the pigment has be-
come generally deposited in the choroid, a colourless streak
marking out the position of the choroidal fissure is very con-
spicuous.
In front of the optic cup the mesoblastic investment
grows forwards, between the lens and the superficial epi-
blast, and so gives rise to the substance of the cornea; the
epi blast supplying only the anterior epithelium.
At first the whole space between the lens and the super-
ficial epiblast is occupied by undifferentiated mesoblast; but
on the sixth day a layer of epithelium makes its ap-
pearance in midst of the mass, and thus divides it into an
anterior and a posterior portion. The anterior portion, in-
creasing in solidity, becomes the cornea, and remains con-
tinuous with the sclerotic; the epithelium in question per-
sisting as the posterior epithelium of the membrane of
Descemet. The posterior portion is reduced to a mere
membrane forming, according to Lieberkiihn, the front
limb of the capsule (and the suspensory ligament) of the
lens, the space between it and the cornea becoming filled
with aqueous humour.
We left the original cavity of the primary optic vesicle as
V.] THE OPTIC CUP. 103
a nearly obliterated space between the two walls of the optic
cup. By the end of the third day the obliteration is com-
plete, and the two walls are in immediate contact.
The inner or anterior wall is, from the first, thicker than
the outer or posterior ; and over the greater part of the cup
this contrast increases with the growth of the eye, the
anterior wall becoming markedly thicker and undergoing
changes of which we shall have to speak directly (Fig. 31).
In the front portion however, along, so to speak, the
lip of the cup, anterior to a line which afterwards becomes
the ora serrata, both layers not only cease to take part in
the increased thickening, accompanied by peculiar histo-
logical changes, which the rest of the cup is undergoing,
but also completely coalesce together. Thus a hind portion
or true retina is marked off from a front portion.
The front portion, accompanied by the choroid which
immediately overlays it, is behind the lens thrown into folds,
the ciliary ridges ; while further forward it bends in between
the lens and the cornea to form the iris. The original wide
opening of the optic cup is thus narrowed to a smaller
orifice, the pupil ; and the lens, which before lay in the open
mouth of, is now inclosed in the cavity of the cup. While in
the hind portion of the cup or retina proper, no deposit of
black pigment takes place in the layer formed out of the
inner or anterior wall of the vesicle, in the front portion we
are speaking of, pigment is largely deposited throughout both
layers, so that eventually this portion seems to become
nothing more than a forward prolongation of the pigment-
epithelium of the choroid.
Thus while the hind moiety of the optic cup becomes the
retina proper, including the choroid-pigment in which the
rods and cones are imbedded, the front moiety is converted
into the ciliary portion of the retina, covering the ciliary
processes, and into the uvea of the iris ; the bodies of the
ciliary processes and the substance of the iris, their vessels,
muscles, connective tissue and ramified pigment, being derived
from the mesoblastic choroid. The margin of the pupil marks
the extreme lip of the optic vesicle, where the outer or poste-
rior wall turns round to join the inner or anterior.
We have still to speak of the choroidal fissure. In
mammals the slit remains open for a short time only. After
104
THE THIRD DAY.
FIG. 31.
[CHAP.
p.Ch
SECTION OP THE EYE OF CHICK AT THE FOURTH DAT.
e. p. superficial epiblast of the side of the head.
R. true retina : anterior wall of the optic eup. p. Ch. pigment-epithelium of
the choroid : posterior wall of the optic cup. b is placed at the extreme
lip of the optic cup at what will become the margin of the pupil.
/. the lens. The hind wall, the nuclei of whose elongated cells are shewn at
nl, now forms nearly the whole mass of the lens, the front wall being
reduced to a layer of flattened cells el.
m. the mesoblast surrounding the optic cup arid about to form the choroid and
sclerotic. It is seen to pass forward between the lip of the optic cup and
the superficial epiblast.
Filling up a large part of the hollow of the optic cup is seen a hyaline mass
vh, possibly the rudiment of the vitreous humour. It has fallen away from
the retina at #, and is also (apparently accidentally) wanting at y. In the
neighbourhood of the lens it seems to be continuous as at cl with the tissue a,
which in turn is continuous with the mesoblast m, and appears to be the
rudiment of the capsule of the lens and suspensory ligament.
V.] THE RETINA. 105
the formation of the vitreous humour within the cup, the
edges of the slit grow completely together, and all traces of
the seam disappear. In birds the course of events is some-
what different.
In addition to such amount of mesoblast as may pass
through the slit to form the vitreous humour, two special
processes of mesoblast grow in, one in the neighbourhood of
the optic stalk, in the region of the true retina, and a second,
which speedily becomes highly vascular, in that portion of
the slit which corresponds to the ciliary part of the retina.
The former piocess remains as the pecten so characteristic
of the avian eye, while the latter vascular process serves to
supply the pecten with blood.
By the twelfth day the fissure completely closes up and
disappears between these two processes and also in front of
the vascular one; but both the pecten and the vascular process
are left projecting into the interior of the eye. Hence in the
adult eye, the pecten seems to perforate the retina close to
the entrance of the optic nerve, the nervous fibres of the
retina spreading away in a radiate manner from it.
The optic stalk, which, as we have said, by an obliteration
of its central canal becomes converted into the optic nerve, is
at first equally continuous with the inner and with the outer
wall of the retina. This must of necessity be the case, since
the interval which primarily exists between the two walls
is continuous with the cavity of the stalk (vide Figs. 28
and 30 F, s). When the fibres however make their appear-
ance in the nerve, they are found to be connected with the
inner wall, or functional retina, only.
The histological condition of the eye in its earliest
stages is very simple. Both the epiblast forming the walls of
the optic vesicle, and the superficial layer which is thickened
to become the lens, are composed of several layers of simple
columnar cells. The surrounding mesoblast is made up of
cells whose protoplasm is more or less branched and irregu-
lar. These simple elements are gradually modified into the
complicated tissues of the adult eye, the changes undergone
being most marked in the cases of the retina, the optic nerve,
and the lens with its appendages.
The retina. At first the two walls of the optic cup do not
greatly differ in thickness. On the third day the outer or
106 THE THIRD DAY. [CHAP.
posterior becomes much thinner than the inner or anterior,
and by the middle of the fourth day is reduced to a single
layer of flattened cells (Fig. 31, p. Ch.). At about the 80th
hour its cells commence to receive a deposit of pigment, and
eventually form the so-called pigmentary epithelium of the
choroid ; from them no part of the true retina (or no other
part of the retina, if the pigment-layer in question be sup-
posed to belong more truly to the retina than to the choroid)
is derived.
On the fourth day, the inner (anterior) wall of the optic
cup (Fig. 31, E) is perfectly uniform in structure, being
composed of elongated somewhat spindle-shaped cells, with
distinct nuclei. On its external (posterior) surface a distinct
cuticular membrane, the membrana limitans externa, early
appears.
As the wall increases in thickness, these cells multiply
rapidly, so that the wall becomes several cells thick.
The first indications of a division into layers are noticed
on the seventh day ; and on the eighth day a layer of
'granules' is very obvious. The granules, which are appar-
ently nuclei of cells, become on the tenth day distinctly
arranged into an inner and an outer layer ; and at about
the same time small processes, apparently outgrowths from
the outer granular layer, make their appearance on the
external surface of the membrana limitans externa. These
processes are the rudiments of the rods and cones.
From the first they may be roughly divided into two categories, (i) those of
smaller, (2) those of larger diameter. Both kinds grow rapidly and in the tips
of both small highly refractive globules soon appear. The thinner processes
are the cones, the thicker the rods. The cones remain for a long time thinner
than the rods, but shortly before the exclusion of the chick they increase rapidly
in diameter and soon after that occurrence are found to surpass the rods in
thickness. On the i8th day some of the globules in the cones become
red, on the iQth others become yellow, and very soon all the globules in the
cones acquire a distinct colour. The globules in the rods remain uncoloured.
The rods and cones then are outgrowths through the membrana limitans externa,
from the inner wa 1 ! of the optic cup or retina into the outer wall or pigment-
epithelium of the choroid.
Remak and some other investigators were of opinion that the outer wall of
the optic cup gave rise to the rods and cones as well as to the pigment-epi-
thelium. The observations however of Max Schultze, Archiv Micros. Anat IV.
]>. 239, supported by Babuchin, Wurz. Nat. Zeifsch. TV. (1863) p. 71, and others,
Lave clearly shewn that Remak's views were erroneous.
On the thirteenth day the molecular layer and the gan-
V.] THE OPTIC NERVE. 107
glionic layer are distinguishable. Very early the substance of
certain of the cells takes on the appearance of fibres, arranged
vertically, i. e. radiating from the inner or anterior surface of
the retina to the membrana limitans externa. These are
the rudiments of the fibres of Miiller.
Thus of the cells of the inner wall of the cup, some
becoine ganglionic cells, and others the fibres of Miiller,
while the nuclei of yet others remain as the inner and outer
granules. The rods and cones are outgrowths of the cells to
which the outer granules belong. All pa.rts of the retina,
in fact, whether simply connective, or really nervous in nature,
seem to be derived from epiblastic cells.
The changes described above are confined to that portion
of the retina which lies behind the ora serrata. In front of
this both walls of the cup coalesce as we have said into a
cellular layer in which a deposit of pigment takes place.
The optic nerve. Histological changes are first observable
in the optic stalk at about the time when its cavity loses all
connection with the cerebral hemisphere and opens ex-
clusively into the third ventricle.. It is then that fibres first
make their appearance in its walls, nuclei being still abun-
dantly present. The stalk though much elongated is still
hollow and its cavity is circular in section. According to
Lieberkiihn at no time does it (in the bird) undergo any
involution tending to obliterate its cavity.
Soon after the deposition of pigment in the outer wall of
the optic cup, while the optic stalks are as yet still hollow,
the rudiments of the optic chiasma appear. The fibres of
the one stalk grow over into the attachment of the other.
About the same time the fibres at the neck of the optic cup
grow forward and become connected with the retina, over whose
internal surface they spread. The entrance of the optic
nerve into the eyeball is closely connected with that of the
pecten, its fibres passing in at the lower end of that body,
coursing along its sides to its upper end and radiating from
it as from a centre to all parts of the retina.
Before the exclusion of the chick the optic nerve becomes
solid by the gradual filling up of its central cavity.
The lens when first formed is somewhat elliptical in
section with a small central cavity of a similar shape,
the front and hind walls being of nearly equal thickness,
108 THE THIRD DAY. [CHAP.
each consisting of a single layer of elongated columnar
cells.
In the subsequent growth of the lens, the development of
the hind wall is of a precisely opposite character to that of
the front wall. The hind wall becomes much thicker, and
tends to obliterate the central cavity by becoming convex on
its front surface. At the same time its cells, still remaining
as a single layer, become elongated and fibre-like. The
front wall on the contrary becomes thinner and thinner and
its cells more and more flattened and pavement-like.
These modes of growth continue until at the end of the
fourth day, as shewn in Fig. 31, the central hind wall I is in
absolute contact with the front wall el and the cavity thus
becomes entirely obliterated. The cells of the hind wall have
by this time become veritable fibres which when seen in
section appear to be arranged nearly parallel to the optic
axis, their nuclei nl being seen in a row along their middle.
The front wall, somewhat thickened at either side where
it becomes continuous with the hind wall, is now a single
layer of flattened ceils separating the hind wail of the lens,
or as we may now say the lens itself, from the front limb of
the lens-capsule ; of this it becomes the epithelium.
The subsequent changes undergone consist chiefly in
the continued elongation and multiplication of the lens-fibres,
with the partial disappearance of their nuclei.
During their multiplication they become arranged in the
manner so characteristic of the adult lens.
The lens-capsule and its adjuncts. In spite of the
numerous investigations which have been made in reference
to the development of the lens-capsule, its precise mode
of origin can hardly even yet be said to be certainly known.
Remak was led from analogy to regard it as a product of the
mesoblast, though he did not succeed in satisfactorily demon-
strating the fact. Kolliker looked upon it as a cuticular
membrane thrown off by the superficial cells of the lens, and
his view has been very generally adopted.
Lieberkiihn has given a different account of its origin.
According to him the involution of the lens, as we have already
stated, carries inwards with it a very thin layer of meso-
blast. This remains continuous with the mesoblast surround-
ing the eyeball, so that when subsequently the mesoblast
V.] THE LENS-CAPSULE. 109
grows forward over the front of the lens, the latter receives
a complete mesoblastic investment.
Of this mesoblast a very thin layer next to the lens
both in front and behind becomes separated from the
rest, and forms the lens-capsule and suspensory ligament.
The remainder of the mesoblast behind the lens becomes
converted into the vitreous humour, the layer immediately
in contact with the retina giving rise to the hyaloid
membrane. That the hyaloid is really a product of the
mesoblast and not a cuticular outgrowth from the epiblastic
cells of the retina is indicated by the fact that it is con-
tinuous over the pecten, where of course the retina is absent.
At its first appearance the vitreous humour is a mass of
stellate cells ; while however it is rapidly enlarging to fill
up the growing optic cup, a large portion of it becomes
entirely fluid, the cellular elements being more and more
restricted to the immediate neighbourhood of the posterior
surface of the lens, where a few stellate cells may be seen
even in the adult.
Briefly to recapitulate. The eye commences as a lateral
outgrowth of the fore-brain, in the form of a stalked vesicle.
The stalk becoming narrowed and subsequently solid, is
converted into the optic nerve.
An involution of the superficial epiblast over the front of
the optic vesicle, in the form first of a pit, then of a closed sac
with thick walls, and lastly, of a solid rounded mass (the small
central cavity being entirely obliterated by the thickening of
the hind wall), gives rise to the lens. Owing to this involu-
tion of the lens, the optic vesicle is doubled up on itself, and its
cavity obliterated ; thus a secondary optic vesicle or optic cup
with a thick anterior and a thin posterior wall is produced. As
a result of the manner in which the doubling up takes place,
or of the mode of growth afterwards, the cup of the secondary
optic vesicle is at first imperfect along its under surface, where
a gap, the choroidal fissure, exists for some little time, but
subsequently closes up.
The mesoblast in which the eye is imbedded gathers
itself together around the optic cup into a distinct invest-
ment of which the internal layers become the choroid, the
external, the sclerotic. An ingrowth of this investment
between the front surface of the lens and the superficial
110
THE THIRD DAY.
[CHAP.
epiblast furnishes the body of the cornea, the epiblast itself
remaining as the anterior corneal epithelium.
A portion of mesoblast, carried in from the front by the
lens during its involution, gives rise to the capsule of the
lens and suspensory ligament, while some mesoblast entering
on the under side through the choroidal fissure becomes (in
birds) the pecten, and probably also contributes to the
vitreous humour.
Of the walls of the optic cup, the thinner outer (posterior)
wall becomes, behind the line of the ora serrata, the pigment-
epithelium of the choroid, while the thicker inner (anterior)
wall supplies all the elements of the retina, including the
rods and cones which grow out from it into the pigment-
epithelium.
FIG. 32.
AOA
SECTION THROUGH THE HIND-BRAIN OP A CHICK AT THE END OF THE THIRD
DAY OF INCUBATION.
IV. Fourth ventricle. The section shews the very thin roof and thicker sides
of the ventricle.
Ch. Notochord (diagrammatic shading).
CV. Anterior cardinal vein.
CO. Involuted auditory vesicle. CC points to the end which will form the
cochlear canal. ML. Kecessus labyrinthi. Jiy. Hypoblast lining the
alimentary canal. AO, AOA, Aorta, and aortic arch.
V.] THE EAR. Ill
In front of the line of the ora serrata, both walls of the
optic cup, quite thin and wholly fused together, give rise to
the pigment-epithelium of the ciliary processes and iris, the
bodies of both these organs being formed from the meso-
blastic investment.
12. During the second day the ear first made its appear-
ance, on either side of the hind-brain as an involution of the
external epi blast, thrust down into the mass of mesoblast
rapidly developing between the epiblast of the skin and that of
the neural canal (Fig. -15, au.p.). It then had the form of a
shallow pit with a widely open mouth. Before the end of the
third day, its mouth closes up and all signs of the opening are
obliterated. The pit thus becomes converted into a closed
vesicle lined with epiblast and surrounded by mesoblast.
This vesicle is the otic vesicle, whose cavity rapidly enlarges
while its walls become thickened (Fig. 32, (7(7).
The changes by which this simple otic vesicle is converted
into the complicated system of parts known as the internal
ear, have been much more completely worked out for
mammals than for birds. We shall therefore reserve a full
account of them for a later portion of this work. Meanwhile a
brief statement of the main course of events in the chick may
be useful ; and will be most conveniently introduced here,
although we shall have, in doing so, to speak of changes
which do not occur till much later than the third day.
The internal ear consists essentially of an inner mem-
branous labyrinth lying loosely in and only partially attached
to an outer osseous labyrinth.
The membranous labyrinth (Fig. 33) consists of two
parts: (1) the vestibule, with which are connected three pairs
of semicircular canals, pag', fr, hor, and a long narrow
hollow process, the aqueductus or recessus vestibuli, and
(2) the ductus cochlearis, which in birds is a flask-shaped
cavity slightly bent on itself. The several parts of each of
these cavities freely communicate, and the two are joined
together by a narrow canal, the canalis reuniens, cr.
The osseous labyrinth has a corresponding form, and may
be similarly divided into parts : into a bony vestibule, with
its bony semicircular canals and recessus vestibuli, and into
a bony cochlea ; but the junction between the cochlea and the
bony vestibule is much wider than the membranous canalis
reuniens.
112
THE THIRD DAY.
FIG. 33.
B
[CHAP.
-pay'
TWO VIEWS OP THE MEMBRANOUS LABYRINTH OF COLUMBA DOMESTICA (copied
from Hasse). A from the exterior, B from the interior.
hoi'' horizontal semicircular canal, hor ampulla of ditto, pag' posterior vertical
semicircular canal, pag ampulla of ditto, /?' anterior vertical semicircular
canal, fr ampulla of ditto, u utriculus, ru recessus utriculi, v the connect-
ing tube between the ampulla of the anterior vertical semicircular canal and
the utriculus, de ductus endolymphaticus (recessus vestibuli), s sacculus
hemisphericus (this is smaller in birds than in any other vertebrate), cr
canalis reuniens, lag lagena (the dilated extremity of the cochlea), mr
membrane of Reissner, which forms the boundary between the scala
vestibuli and scala media, pb Basilar membrane, which forms the boundary
between the scala tympani and the scala media.
The cochlea of a bird consists ( r) of a scala vestibuli with a very small lumen,
which opens at one end into the perilymphatic cavity of the vestibule, and at
the other into the lagena (the dilated extremity of the cochlea corresponding
with the cupola of mammals) ; (21 of a scala tympani, also opening into the
lagena at one end, and into the foramen rotundum at the other; (3) of a
scala media ending blindly at one end, but in communication with the mem-
branous vestibule at the other, through the membranous canalis reuniens (cr).
As in mammals, the cavity of the osseous cochlea is
divided lengthways by the ductus cochlearis into a scala
tympani ending in a foramen rotundum, and a scala vestibuli
ending in the cavity of the osseous vestibule, which in its
turn is connected with the foramen ovale.
The auditory nerve, piercing the osseous labyrinth in
various points, is distributed in the walls of the membranous
labyrinth.
All these complicated structures are derived from the
simple primary otic vesicle by changes in its form and
differentiation of its walls. All the epiblast of the vesicle
THE MEMBRANOUS LABYRINTH.
113
goes to form the epithelium of the membranous labyrinth,
whose curiously twisted cavity filled with endolymph repre-
sents the original cavity which was first open to the surface
but subsequently covered in. The corium of the mem-
branous labyrinth, and all the tissues of the osseous labyrinth,
FIG. 34.
R.Z7
V:B
TRANSVERSE SECTION OF THE HEAD OF A FCETAL SHEEP (16 MM. IN LENGTH)
IN THE REGION OF THE HIND BRAIN. (Copied from Boettcher, Entwicke-
lung und Bau des Gehorlabyrinths.)
This figure, together with Figs. 33 and 34, though referring to mammalian
structures, are introduced here, in order to illustrate the account given in the
text.
H.B. the hind brain, the lines of reference starting on both sides from the
greatly thickened side walls.
The section is somewhat oblique, hence while on the right side the connec-
tions of the recessus vestibuli, R.L., and of the commencing vertical semicircular
canal V.B. and of the ductus cochlearis C. C., with the cavity of the primary
otic vesicle are well seen, on the left side, only the extreme end of the ductus
cochlearis C.C. and of the semicircular canal V.B. are shewn. In the same way
the cavity of the throat appears from the obliquity of the section to be one-sided.
Lying close to the inner side of the otic vesicle is seen the cochlear
ganglion G. C. ; on the left side the auditory nerve G and its connection N with
the hind-brain are also shewn.
Below the otic vesicle on either side lies the cardinal vein.
E.
8
THE THIRD DAY.
[CHAP.
are developed out of the mesoblastic investment of the
vesicle, the (lymphatic) space between the osseous and
membranous labyrinths being hollowed out of the mesoblast,
and becoming filled with perilymph as it is formed. The chief
stages are as follows :
The form of the vesicle as seen in transverse sections
of the head from being oval becomes somewhat triangular
with the apex directed downwards (i. e. towards the ventral
surface of the body). (Fig. 32.)
The apex continues to develope until it becomes a some-
what pointed process directed inwards (Fig. 34, (7(7), and lying
somewhat in front (nearer the face than) the rest of the vesicle,
FIG. 35.
c.c
SECTION OF THE HEAD OF A FOETAL SHEEP (20 MM. IN LENGTH).
(Copied from Boettcher.)
?. V. Recessus Vestibuli. V.B. Vertical semicircular canal. H.B. Horizontal
semicircular canal. C.C. Cochlear canal. G. Cochlear ganglion.
V.] THE SEMICIRCULAR CANALS. 115
from which it soon becomes definitely marked off by a constric-
tion as the ductus cochlearis (Fig. 35, C. (7.), leaving the rest of
the vesicle to form the vestibule and its appendages. About the
same time that the ductus cochlearis is developing the upper
and outer corner of the triangular vestibule grows out up-
wards and backwards, as a long hollow process, the recessus
or aqueductus vestibuli (Fig. 35, -RF, 36, RV), marked
where it leaves the vestibule by a more* or less prominent
constriction. The upper end of this is the remnant of the
primitive opening of the auditory involution.
On the remaining surface of the vestibule two hollow
protuberances are visible (Fig. 34, V.B.], the rudiments of
the two vertical semicircular canals. Below this, a little later
on, a similar bulging starts to form the horizontal semi-
circular canal (Fig. 35, H.B.). Each protuberance grows
out, becomes flattened, and while remaining attached to the
vestibule at its two ends becomes separated from it in the
middle, and is thus converted into a single tube opening at
both ends (one of which subsequently dilates into an ampulla)
into the cavity of the vestibule.
Near the junction of the ductus cochlearis with the
vestibule, two constrictions give rise to a prominence lying
between them, and known as the sacculus hemisphericus. This,
though very conspicuous in mammals (Fig. 36, S.B.}, is very
slightly marked in birds (Fig. 33, s). The rest of the vesti-
i bule remains as the utriculus. The progressive constriction
I at the base of the ductus cochlearis gives rise to the canalis
i reuniens. Thus, by an unequal growth resulting in these
protuberances and constrictions, the originally oval sac is
I modelled into the membranous labyrinth.
While the vesicle is thus increasing in size and changing
in form, a large quantity of mesoblast is gathered round it.
The outer portion of this mesoblastic investment becoming
dense is converted into cartilage, while the internal portion
remains as undifferentiated tissue. Later on, the fnost in-
ternal layers of this undifferentiated tissue are converted
into the membranous coat (corium) which supports the
epithelium of the membranous labyrinth ; while the re-
mainder is liquefied or absorbed at the same time that the
cartilaginous envelope becomes ossified, and so gives rise to
the cavity of the osseous labyrinth with its contained peri-
116
THE THIRD DAY.
FIG. 36.
[CHAP.
C/i-
ac
SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP (28 MM.
IN LENGTH). (Copied from Boettcher.)
DM. Dura Mater. R.V. Recessus vestibuli. H.V.B. Posterior vertical
semicircular canal. U. Utriculus. H.B. Horizontal semicircular canal.
b. Canalis reuniens. a. Constriction by means of which the sacculus
hemisphericus S.R. is formed. /. Narrowed opening between sacculus
hemisphericus and utriculus. 0.0. Cochlea. <7.<7. Lumen of cochlea.
K.K. Cartilaginous capsule of cochlea. K.B. Investing mass. Oh.
Chorda dorsalis.
V.] THE NASAL PITS. 117
lymph. In the region of the cochlea this excavation of the
mesoblast takes place along two definite tracts, and thus the
two scalse are established ; but in the region of the vestibule
and its appendages it is more irregular, resulting in one
common cavity interrupted at various points by bridles of
connective tissue passing from the osseous to the membranous
labyrinth.
Further details, especially concerning the histological
changes, we propose to reserve for a later part of this
work. Meanwhile we may remark that all the minute
auditory apparatus, the hair-cells, and their various adjuncts,
appear to be of a distinctly epithelial nature and of epiblastic
origin. In the bird, as is well known, there are no rods of
Corti; but even these structures seem in the mammal to be
similarly epiblastic.
It will be seen then that the ear, while resembling the
eye in so far as the peculiar structures in which the sensory
nerve in each case terminates are formed of involuted epi-
blast, differs from it inasmuch as it arises by an independent
involution of the superficial epiblast, whereas the eye is a
constricted portion of the general involution which gives rise
to the central nervous system. Still greater is the distinction
between the optic and auditory nerves. The optic nerve is,
as we have seen, the consolidated stalk of the optic vesicle,
and therefore is of purely epiblastic origin. The auditory
nerve, on the contrary, as we shall see, arises in and is formed
out of the mesoblast, lying by the side of the otic vesicle. It
with its ganglion may readily be recognised in sections as
quite independent, both of the otic vesicle and the hind-
brain, though subsequently coming into connection with
each of them. The growth of the auditory nerve into the
hind-brain is shewn in Fig. 34, N; the union of the nerve-
fibres with the epithelial structures of the membranous
labyrinth takes place at a later date.
13. At the under surface of each of the vesicles of the
cerebral hemispheres there appears towards the end of the
third day a small spmewhat elongated vesicle, the olfactory
vesicle, which is the rudiment of the olfactory nerve or bulb.
Over each olfactory vesicle the external epiblast which covers
them grows inwards to form a shallow depression with a
thickened border. These depressions are the nasal pits (Fig.
118 THE THIRD DAY. [CHAP.
37, N). Like the lens and the labyrinth of the ear, they are
HEAD OF AN EMBRYO CHICK OF THE THIRD DAY VIEWED SIDEWAYS AS AN
OPAQUE OBJECT. (Chromic acid preparation.)
C.H. Cerebral hemispheres. F. B. Vesicle of third ventricle. M.B. Mid-brain.
Cb. Cerebellum. H.B. Medulla oblongata.
N. Nasal pit. ot. otic vesicle in the stage of a pit with the opening not yet
closed up. op. Optic vesicle, with I. lens and ch.f. choroidal fissure. The
small dot in the centre of the lens indicates the remnant of its external
opening. The superficial epiblast moulds itself to the form of the optic
vesicle and the lens ; hence the choroidal fissure, though formed entirely
underneath the superficial epiblast, is distinctly visible from the outside.
T F. The first visceral fold ; above it is seen a slight indication of the superior
maxillary process.
2, 3, 4 F. Second, third and fourth visceral folds, with the visceral clefts
between them.
SO. Portion of the somatopleure springing from between the ends of the
visceral folds.
formed from the external epiblast; unlike them they are
never closed up. At first they have no distinct connection
with their respective olfactory vesicles, and their openings
are independent and separate, there being as yet no actual
mouth to connect them with each other.
14. It must be borne in mind that especially in the
early stages of development, owing, to the very unequal
growth of different parts, the relative position of the various
structures is continually shifting. This is very well seen in
the instance of the heart. At its first appearance, the heart
is lodged immediately beneath the extreme front of the ali-
mentary canal so far forwards as to underlie that portion of
the medullary canal which will form the brain. It is, in
fact, at that epoch a part of the head. From that early
position it gradually recedes farther and farther backward,
until, at the end of the third day, a considerable interval is
V.] THE VISCERAL CLEFTS. 119
observed between it and the actual head. In other words,
a distinct neck has been formed, in which most important
changes take place.
The neck is distinguished from the trunk in which the
heart now lies by the important feature that in it there is no
cleavage of the mesoblast into somatopleure and splanch-
nopleure, and consequently no pleuroperitoneal cavity. In
passing from the exterior into the alimentary canal, the three
layers of the blastoderm are successively traversed without
any breach of continuity save such as is caused by the cavities
of the blood-vessels. In this neck, so constituted, there ap-
pear on the third day certain fissures or clefts, the visceral or
branchial clefts. These are real clefts or slits passing right
through the walls of the throat, and are placed in series on
either side across the axis of the alimentary canal, lying not
quite at right angles to that axis and parallel to each other,
but converging somewhat to the middle of the throat in front
(Fig. 37). Viewed from the outside in either fresh or pre-
served embryos they are not very distinctly seen to be clefts;
but when they are seen from within after laying open the
throat, their characters as elongated oval slits can easily be
recognised.
Four in number on either side, the most anterior is the
first to be formed, the other three following in succession.
Their formation takes place from within outwards. The
hypoblast and mesoblast are first absorbed along the line
of the future cleft, then the epiblast is broken through, and
the hypoblast, which is carried outwards as a lining to the
slit, forms a junction with the epiblast at the outside of the
throat.
No sooner has a cleft been formed than its upper border
(i. e. the border nearer the head) becomes raised into a thick
lip or fold, the visceral or branchial fold. Each cleft has its
own fold on its upper border, and in addition the lower
border of the fourth or last visceral cleft is raised into a
similar fold. There are thus five visceral folds to four
visceral clefts (Fig. 37). The last two folds however, and
especially the last, are not nearly so thick and prominent
as the other three, the second being the broadest and most
conspicuous of all. The first fold meets, or nearly meets its
fellow in the middle line in front, but the second falls short
120 THE THIRD DAY. [CHAP.
of reaching the middle line, and the third, fourth and fifth
do so in an increasing degree. Thus in front views of the
neck a triangular space with its apex directed towards the
head is observed between the ends of the several folds.
Into this space the pleuroperitoneal cavity extends, the
somatopleure separating from the splanchnopleure along the
ends of the folds; and it is here that the aorta plunges into
the mesoblast of the body.
Of the history of these most important visceral folds and
clefts we shall speak in detail hereafter ; meanwhile we may
say that in the chick and higher vertebrates the first three
pairs of folds are those which call for most notice.
The first fold on either side, increasing rapidly in size
and prominence, does not, like the others, remain single, but
sends off in the course of the third day a branch or bud-like
process from its upper edge. This branch, starting from
near the outer beginning of the fold, runs forwards and
upwards, tending to meet the corresponding branch from the
fold on the other side, at a point in the middle line nearer
the front of the head than the junction of the main folds.
The two branches do not quite meefc, being separated by
a median process, which at the same time grows down
from the extreme front of the head, and against which they
abut. Between the main folds, which are directed somewhat
downwards and the branches which slant upwards, a some-
what lozenge-shaped space is developed which, as the folds
become more and more prominent, grows deeper and deeper.
The main folds are the rudiments of the inferior maxillcv,
the branches those of the superior maxillce, the lozenge-
shaped cavity between them is the cavity of the mouth,
and the descending process which helps to complete the
upper margin of this cavity is called, from the parts which
will be formed out of it, the fronto-nasal process.
Already on the second day the under wall of the front
end of the alimentary canal began to become thin, while over
it the epiblast was pushed in so as to form a depression. The
maxillary folds convert this depression into a deep pit,
whose bottom is not as yet perforated, the opening into
the alimentary canal being made later on.
The two succeeding pairs of visceral folds are transformed into parts which
will be best considered in connection with the development of the skull. The
V.] THE AORTIC ARCHES. 121
last two disappear in the chick without giving rise to any permanent
structures.
The first visceral cleft remains permanently open, but is drawn out by the
growth of surrounding parts into a long tube, subsequently divided into the
rneatus auditorius and the Eustachian tube. The other visceral cleits are
obliterated.
15. By the end of the second day three pairs of aortic
arches had been established in connection with the heart.
When the visceral folds and clefts are formed, a definite
arrangement between them and the aortic arches is always
FIQ. 38.
THE SAME HEAD AS SHEWN IN FIG. 37, SEEN FROM THE FRONT.
The neck has been cut across between the first and second visceral folds, the
incision being carried through the first visceral cleft. In the cut surface b
are seen the sections of the hind brain Hb., and of blood-vessels c.
i F. The first visceral fold ; between the ends of the fold is seen a section of
the somatopleure at its extreme forward limit; in it lies the aorta .
Below the folds is the cavity of the throat a/, and /. Fis placed in the first
visceral cleft.
C.H. Cerebral hemispheres. N. Nasal pit. ch.f. groove indicating the
choroidal fissure.
observed. The first visceral cleft runs between the first and
second aortic arches. Consequently the first aortic arch runs
in the first visceral fold and the second in the second. In
the same way, the second visceral cleft lies between the
second and third aortic arches, the third aortic arch running
in the third visceral fold. Each aortic arch runs in the
thickened mesoblast of the corresponding fold.
Arrived at the upper surface of the alimentary canal,
these arches unite at acute angles to form a common trunk, the
dorsal aorta (Fig. 39 A, A. 0\ which runs along the back imme-
diately under the notochord. The length of this common single
trunk is not great, as it soon divides into two main branches
122
[CHAP.
DIAGRAM OF THE ARTERIAL CIRCULATION ON THE THIRD DAY.
I, 2, 3. The first three pairs of aortic arches. A . The vessel formed byythe
junction of the three pairs of arches. A.O. Dorsal aorta formed by the
junction of the two branches A and A ; it quickly divides again into two
branches which pass down one on each side of the notochord. From each
of these, not far from its termination, is given off a large branch Of. A.,
the omphalo-mesaraic artery.
(the future common iliacs), each of which, after giving off the
large omphalo-mesaraic artery, Of. A., pursues its course with
diminished calibre to the tail, where it is finally lost in the.
capillaries of that part.
16. The heart is now completely doubled up on itself.
Its mode of curvature is apparently somewhat complicated.
V.] THE HEART. 123
Starting from the point of junction of the omphalo-mesaraic
veins (Fig. 24, Ht), there is first a slight curvature towards the
left ; this is followed by a turn to the right, and then the heart
is completely bent on itself, so that afterwards it pursues a
course directed from behind quite straight forwards (except
perhaps for a little inclination to the left) to the point where
the aortic arches branch off. In this way the end of the
bulbus arteriosus comes to lie just underneath (or in front of
according to the position of the embryo) that part which has
already been marked off by the lateral bulgings as the
auricular portion.
That part of the heart which is turned to the right,
including the point of doubling up, is the ventricular portion,
and is even at this stage separated from the auricular portion
by a slight neck. This external constriction corresponds to
an internal narrowing of the lumen of the heart, and marks
the position of the future canalis auricularis.
The ventricular portion is, on the other hand, likewise
separated by a fainter constriction from the anterior continua-
tion of the heart which forms the bulbus arteriosus. The
projecting part where the doubling takes place is at this
stage still quite round ; we shall see that later on it becomes
pointed and forms the apex of the heart.
The whole venous portion of the heart (if we may so
speak, though of course at this stage blood of the same
quality passes right along the whole cardiac canal) lies in a
plane which is more dorsal than the arterial portion. The
point at which the venous roots of the heart, i. e. the two
omphalo-mesaraic trunks unite into a single canal, is on this
day carried farther and farther away from the heart itself.
By the end of the day there is a considerable distance
between the auricular portion of the actual heart and the
point where the venous roots separate each to pursue its
course along the splanchnopleure-fold of its own side. This
distance is traversed by a single venous trunk, of which the
portion close to the auricles is called the sinus venosus, and
the more distant the ductus venosus. We shall give to the
whole trunk the name used by the older observers, the
meatus venosus.
Small arteries to various parts of the body are now
being given off by the aorta and its branches. The capil-
124
THE THIRD DAY.
FIG. 39. B.
[CHAP.
DIAGRAM OF THE VENOUS CIRCULATION ON THE THIRD DAT.
H. Heart. D. C. Ductus Cuvieri. S.V. Meatus venosus. Su. V. Superior
vertebral or anterior cardinal vein. C. Inferior or posterior cardinal vein.
Of. Omphalo-mesaraic vein.
laries in which these end are gathered into veins which
unite to form two main trunks on either side, the cardinal
veins, anterior and posterior (Fig. 23, Fig. 39 B, Su. V. and (7.),
which run parallel to the long axis of the body in the upper
part of the mesoblast, a little external to the protovertebrae.
These veins, which have not by the third day attained to any
great importance, unite opposite to the heart, on each side,
into a short common trunk at right angles to themselves.
The two short trunks thus formed, which bear the name of
Ductus Cuvieri (Fig. 23, Fig. 39 B, D.C.), running trans-
versely straight inwards towards the middle line fall into the
sinus venosus.
The blood-vessels in the body of the embryo take their origin from the
mesoblast exclusively, and their formation is probably precisely similar to that of
the vessels in the vascular area, branches being given out from or brought into
connection with the aorta and omphalo-mesaraic veins, in the same way as
branches were described as springing from or meeting the earliest formed vas-
cular channels.
His, carrying out the views we have already referred to, believed that
parablastic elements grew inwards along the omphalo-mesaraic trunks, through
the length of the heart and so onwards into the aorta and all its branches, until
V-]
THE TAIL-FOLD.
125
the whole archiblastic framework of the embryo became riddled by a network
of parablast. From these parablastic elements he considered there arose not
only the epithelium (endothelium) of the blood-vessels and lymphatic spaces,
but also all the connective-tissue elements of the body, the archiblast being
represented in the vessels of the adult body by the muscular fibres alone.
17. As we stated above (p. 87), the folding in of the
splanchnopleure to form the alimentary canal is proceeding
with great rapidity, the tail-fold as well as the head -fold con-
tributing largely to the result.
The formation of the tail-fold is very similar to that of
the head-fold. At the extreme hind end of the embryo, at
the tip of the tail (Fig. 40, i), there is no cleavage of the
mesoblast, and therefore neither somatopleure nor splanchno-
pleure. The tail is a solid, somewhat curved, blunt cone of
mesoblast, immediately coated with the superficial epiblast
FIG. 40.
SECTION or THE TAIL-END OF AN EMBRYO (CHICK) OF THE THIRD DAT. (From
Dobrynin. )
t. the tail. So. somatopleure, Spl. splanchnopleure. pp. pleuroperitoneal space.
The letters G and Dd are placed within the alimentary canal : a more
detailed explanation of the figure is given under Fig. 50.
except at the upper surface (corresponding to the back of the
embryo), where lies the pointed termination of the neural
tube. At some little distance forwards, towards the head,
the cleavage of the mesoblast begins, and the somatopleure
diverges from the splanchnopleure, the latter, as in the head-
fold, being folded in to a greater extent than the former. Ex-
cept for the absence of the cephalic enlargement of the neural
tube, the presence of distinct protovertebrae up to nearly the
126 THE THIRD DAY. [CHAP.
FIG. 41.
SECTION THROUGH THE DORSAL REGION OF AN EMBRYO AT THE COMMENCEMENT
OF THE THIRD DAY.
M. C. medullary canal. Ck. notochord. p. v. protovertebra composed of an
investment of columnar cells enclosing a central nucleus of rounded cells.
w. d. Wolffian duct, which has commenced to travel down from the dorsal
surface of the mesoblast. A. o. dorsal aorta of right side. g. e. germinal
epithelium ; an epithelium of columnar cells lining the upper end of thepleuro-
peritoneal cavity, and which is related to the formation of Miiller's duct and
of the ovary. S. o. somatopleure. s. p. splanchnopleure.
The splanchnopleure is very little folded in, the embryonic sac being in this
middle region widely open to the yolk below. The somatopleure is much more
folded in. At a little distance outside the protovertebra is a ridge with
thickened mesoblast, the Wolffian ridge, marking the line along which the limbs
will be developed. Beyond this ridge the somatopleure suddenly descends to
form the body- wall (of the abdomen) ; it then ascends, and after a fold, probably
due to the action of the chromic acid> forms an angular projection at about the
level of the protovertebra. This projection is the lateral fold of the amnion,
the section having been taken just at that point between the head and tail where
the amnion is, on the third day, least developed.
Beyond the amniotic fold the somatopleure and splanchnopleure come into
apposition, arid still farther out completely coalesce. It will be observed that
the pleuroperitoneal space already reaches laterally far beyond the limits of
the embryo itself.
In the splanchnopleure is seen at V a section of a large branch of the
omphalo-mesaraic trunk.
The shading of the general mesoblast is diagrammatic.
The width of the cavity of the neural tube is unusually great.
V.] THE MESENTERY. 127
actual termination of the tail, and certain features connected
with the development of the allantois, of which we shall
speak presently, the tail is a counterpart of the head.
So rapid is the closure of the splanchnopleure both in
front and behind, that two of the three parts into which
the digestive tract may be divided, are brought, on this day,
to the condition of complete tubes.
The first division, extending from the mouth to the
duodenum, is completely folded in by the end of the day ; so
likewise is the third division comprising the large intestine
and the cloaca. The middle division, corresponding to the
future small intestine, still remains quite open to the yolk-
sac below.
The attachment of the newly formed alimentary canal to
the body above is at first very broad, and only a thin stratum
of mesoblast separates the hypoblast of the canal from the
notochord and proto vertebrae ; even that may be absent under
the notochord (Fig. 41). During the third day, however,
along such portions of the canal as have become regularly
enclosed, i.e. the hinder division and in the posterior moiety
of the anterior division, the mesoblastic attachment becomes
narrower and (in a vertical direction) longer, the canal appear-
ing to be drawn more downwards (or according to the position
of the embryo forwards), away from the vertebral column.
In what may be regarded as the pleura! division of the
general pleuroperitoneal space, along that part of the ali-
mentary canal which will form the oesophagus, this with-
drawal is very slight (compare Fig. 32), but it is very marked
in the peritoneal space. Here such parts of the digestive
canal as are formed come to be suspended from the body
above by a narrow flattened band of mesoblastic tissue which
reaches from the neighbourhood of the notochord, and be-
comes continuous with the mesoblastic coating which wraps
round the hypoblast of the canal. This flattened band is the
mesentery, shewn commencing in Fig. 44, and much more
advanced in Fig. 47, M. It is covered on either side by a
layer of flat cells, while its interior is composed of indifferent
tissue.
The front division of the digestive tract consists of three
parts. The most anterior part, the oesophagus, still ends
blindly in front, and reaches back as far as the level of the
128
THE THIRD DAY.
[CHAP
liind end of the heart. Its transverse section, which up to
the end of the second day was somewhat crescent-shaped, with
the convexity downwards, becomes on this day more nearly
circular. Close to its hinder limit, the lungs (Fig. 42, Ig), of
whose formation we shall speak directly, take their origin.
FIG. 42.
DIAGRAM OP A PORTION OF THE DIGESTIVE TRACT OP A CHICK UPON THE
FOURTH DAY. (Copied from Gotte.)
The black inner line represents the hypoblast, the outer shading the mesoblast.
Iff. lung-diverticulum with expanded termination, forming the primary
lung- vesicle. St. stomach. I. two hepatic diverticula with their terminations
united by solid rows of hypoblast cells, p. diverticulum of the pancreas
with the vesicular diverticula coming from it.
The portion of the digestive canal which succeeds the
oesophagus, becomes towards the close of the third day
somewhat dilated (Fig. 42, St) ; the region of the stomach is
thus indicated.
The hinder or pyloric end of the stomach is separated by
a very small interval from the point where the complete
closing in of the alimentary canal ceases, and where the
splanchnopleure-folds spread out over the yolk. This short
tract is nevertheless clearly marked out as the duodenum by
the fact that from it, as we shall presently point out, the
rudiments of ducts of the liver and pancreas are beginning to
be formed. It forms the third part of the front division.
The posterior division, corresponding to the great intestine
and cloaca, is from its very first formation nearly circular in
V.] THE LUNGS. 129
section and of a larger bore than the oesophagus. Up to the
end of the day it is still completely closed at the hinder
extremity, which however is somewhat swollen to form the
cloaca.
18. The lungs are in origin essentially buds or processes
from the primitive oesophagus.
If the alimentary canal of the chick at the end of the
third day be dissected out and laid open, there will be seen
on either side of the hinder end of the oesophagus a short
pouch or diverticulum enveloped in a mass of mesoblast
(Fig. 42, Ig. ; here however they are somewhat advanced,
the specimen belonging to the fourth day). These pouches
are the early rudiments of the lungs. Their mode of origin
is as follows.
FIG. 43.
FOUK DIAGKAMS ILLUSTRATING THE FOEMATION OF THE LUNGS.
(Copied from Gotte.)
a. mesoblast. 1. hypoblast. d. cavity of digestive canal. I. cavity of the
pulmonary diverticulum.
In (i) the digestive canal has commenced to be constricted into an upper
and lower canal; the former the true alimentary canal, the latter the pulmonary
tube; the two tubes communicate with each other in the centre.
In (2) the lower (pulmonary) tube has become expanded.
In (3) the expanded portion of the tube has become constricted into two
tubes, still communicating with each other and with the digestive canal.
In (4) these are completely separated from each other and from the digestive
canal, and the mesoblast has also begun to exhibit externally changes corre-
sponding to the internal changes which have been going on.
E. 9
130 THE THIRD DAY. [CHAP.
At a point immediately behind (or when the embryo is placed
on its face, above) the heart, the cavity of the alimentary canal
is compressed laterally, and at the same time constricted in
the middle so that its transverse section (Fig. 43, 1) is some-
what hour-glass-shaped, and shews an upper or dorsal cham-
ber d, joining on to a lower or ventral chamber I by a short
narrow neck. Th& lower chamber then becomes broader
than high (Fig. 43, 2) while its under wall is raised up in a
median fold, which partially divides the chamber into two
lateral parts (Fig. 43, 3). As a result of these folds, the
original simple tube becomes divided into three grooves or
incomplete tubes, whose cavities all communicate with each
other at the centre of the original canal : into a single tube
above, which is the true oesophagus, and into two tubes
below, each of which is a rudiment of a lung. The presence
of these three incomplete tubes may be traced, by sections
at various levels, for a certain distance along the alimentary
canal, and are then lost, the canal once more returning to
the condition of a simple tube.
The median fold or partition then rises up so as com-
pletely to shut off the three cavities from each other (Fig.
43, 4). The isolation commences behind and travels thence
forwards, but never quite reaches the point in front where
the division into three cavities begins. As a consequence
the lower pulmonary tubes, though closed behind, open into
the oesophagus in front. In other words, by the constriction
and median folds which we have described, the two short
pouches or diverticula of the lungs are developed from the
under (or anterior) surface of the oesophagus.
At their first origin both diverticula together with the
alimentary canal itself are invested in one common rounded
mass of mesoblast whose external contour bears no marks of
the internal changes which are going on. By and bye, as
the diverticula diverge behind from the median line, they
carry the mesoblast with them (Fig. 43, 4), Then for the
first time they become evident from the outside.
The subsequent history of these diverticula may with convenience be briefly
described here.
At first the two diverticula have separate openings into the oesophagus ; but
as the point at which they enter is carried further forwards (or rather as they in
the process of growth are carried backwards), they unite near their base to form
V.] THE LIVER. 131
a common tube opening on the under surface of the oesophagus. This tube is
the trachea.
At the end of each of the primary diverticula is a small vesicle which may
be called the primary lung-vesicle. It appears ultimately to become the abdo-
minal air-sac.
The mesoblast round the primary diverticula becomes greatly thickened.
Into it secondary diverticula, still lined by hypoblast, penetrate; these again
give rise to tertiary branches and thus the bronchial tubes are formed. At right
angles'to the finest of these the arborescent branches so characteristic of the avian
lung are given off. The whole pulmonary structure is therefore the result of the
growth by budding of a system of branched hypoblastic tubes in the midst of a
mass of mesoblastic tissue, the hypoblastic elements giving rise to the epithelium
of the tubes and the mesoblast providing the elastic, muscular, cartilaginous,
connective and other tissues of the tracheal and bronchial walls.
The air-sacs are primarily the dilated ends of the primitive diverticula or of
their main branches. At first there are three air-sacs on each side : one abdo-
minal (the end of the primitive diverticulum), another thoracic, and a third
extra-thoracic. An additional thoracic, and an additional extra -thoracic, making
in all five air-sacs on each side, appear at a later period.
The pulmonary blood-vessels penetrate into the mesoblast surrounding the
bronchial tubes on about the iith day.
19. The liver is the first formed chylopoietic appendage
of the digestive canal, and arises between the 55th and 60th
hour as a couple of diverticula one from either side of the
duodenum immediately behind the stomach (Fig. 42, I.).
These diverticula, though composed of both hypoblast and
mesoblast, are, according to Gotte, in the first instance solid,
and only subsequently become hollow. The right one is,
in all cases, from the first longer, but of smaller diameter
than the left. Situated a little behind the heart, they
embrace between them the meatus venosus or united trunk
of the omphalo-mesaraic veins, which as it passes between
them exhibits numerous small bulgings. As yet the vein
arid the diverticula, though in close contact, are not connected.
Towards the end of the third day there may be observed
in the greatly thickened mesoblastic investment of either
diverticulum a number of cylindrical solid aggregations of
cells connected with, and apparently outgrowths from, the
hypoblast of the diverticulum. These cylinders rapidly
increase in number, apparently by division, their somewhat
swollen peripheral extremities come into contact and unite.
And thus, about the ninetieth hour, a sort of network of solid
thick strings of hypoblastic cells is formed, the mesoblast in
the meshes of the network becoming at the same time largely
converted into blood-vessels. In addition to this network of
solid hypoblastic cylinders, the diverticula also send out
92
132 THE THIRD DAY. [CHAP.
hollow processes, lined with hypoblast. Each diverticulum
becomes in this way surrounded by a thick mass composed
partly of solid cylinders, and to a less extent of hollow
processes, continuous with the cylinders on the one hand, and
the main diverticulum on the other, all knit together with
commencing blood-vessels and unchanged mesoblastic tissue.
Between the two masses runs the meatus venosus, with the
bulgings on which, referred to above, the blood-vessels in
each mass are connected.
Early on the fourth day each mass sends out underneath
the meatus venosus a solid projection of hypoblastic cylinders
towards its fellow, that from the left side being much the
longest. The two projections unite and form a long solid
wedge, which passes obliquely down from the right (or from
the embryo lying on its left side, the upper) mass to the
left (or lower) one. In this new wedge may be seen the
same arrangement of a network of hypoblastic cylinders
filled in with vascular mesoblast as in the rest of the
liver. The two original diverticula with their investing
masses represent respectively the right and left lobes of the
liver, and the wedge-like bridge connecting them is the
middle lobe.
During the fourth and fifth days the growth of the solid,
lobed liver thus formed is very considerable ; the hypoblastic
cylinders multiply rapidly, and the network formed by them
becomes very close, the meshes containing little more than
blood-vessels. The hollow processes of the diverticula also
ramify widely, each branch being compo3ed of a lining of
hypoblast enveloped in a coating of spindle-shaped meso-
blastic cells. The blood-vessels are in direct connection
with the meatus venosus have become, in fact, branches of
it. It may soon be observed, that in those vessels which are
connected with the posterior part of the liver (Fig. 53),
the stream of blood is directed from the meatus venosus into
the network of the liver. In those connected with the anterior
part the reverse is the case ; here the blood flows from the
liver into the meatus venosus. The thick network of solid
cylinders represents the hepatic parenchyma of the adult
liver, while the hollow processes of the diverticula are the
rudiments of the biliary ducts.
The exact morphological significance of these anastomosing cylinders, and
V.]
THE PANCREAS. 133
the manner of their ultimate metamorphosis into the ordinary hepatic tissue, is
not as yet quite clear. If we suppose each solid cylinder to represent a duct
with its lumen almost, but not quite, completely obliterated, we should gain a
view agreeing very closely with that put forward by Hering on the structure
of the adult liver.
During the fifth day, a special sac or pouch is developed
from the right primary diverticulum. This pouch, consisting
of an inner coat of hypoblast, and an outer of mesoblast, is
the rudiment of the gall-bladder.
20. About the middle of the third day, the pancreas
(Fig. 42, p.) also appears, but its exact mode of origin is still
somewhat doubtful.
According to G-otte (Beit. z. entwicU. des Darmcanals des Huhnchens) it
commences as a thickening of both the hypoblast and mesoblast of a portion of
the wall of the digestive canal on the same level as the left diverticulum of
the liver. The hypoblast in the centre of this thickening becomes hollow,
forming a cavity connected with the inside of the digestive canal by a narrow
opening. Around this cavity processes of hypoblast are seen on the fourth day
stretching into the surrounding mesoblast. These processes, which are at first
solid but afterwards become hollow and ultimately branched, are in the early
stages so completely covered up by mesoblast that they are not visible on the
exterior. The primary cavity elongates into the duct, the hollow processes
representing its branches* On the sixth day a new similar outgrowth takes
place between the primary one and the stomach. This, which ultimately
coalesces with its predecessor, gives rise to the second duct, and forms a
considerable part of the adult pancreas. A third duct is formed at a much
later period. According to this view, with which those put forward by Kolliker
and Remak in the main agree, the so-called 'secreting' cells of the pancreas
as well as the epithelial lining of the ducts are derived from hypoblast.
Schenk (Die Bauchspetiheldriise des Embryos. Anat. u. Physiol. Untersuch-
ung. Wien. S. I.) however is of opinion that the former originate in a
transformation of the mesoblast, the hypoblast giving rise to the epithelium
of the ducts only.
Shortly after the first appearance of the pancreas, the
spleen appears as a thickening of the mesentery of the stomach
(mesogastricum) and is therefore entirely a mesoblastic struc-
ture.
Its development has been recently investigated by Peremeschko (Sitz. der k.
Akad. in .Wien, Bd. 56, 1867) and by W. Miiller (Strieker's Histology).
According to these investigators, the mass of mesoblast which forms the
spleen becomes early separated by a groove on the one side from the pancreas and
on the other from the mesentery. Some of its cells become elongated, and send
out processes which, uniting with like processes from other cells, form the trabe-
cular system. From the remainder of the tissue are derived the cells of the spleen
pulp, which frequently contain more than one nucleus. Especial accumulations
of these take place at a later period to form the so-called Malpighian corpuscles
of the spleen.
21. The thyroid body is also formed towards the end of
the third day, in connection with the alimentary canal.
134 THE THIRD DAY. [CHAP.
According to Miiller (Ueber die Entwickelung der ScTiilddri'ise. Jenaische
Zeitschrift, 1871) who has recently studied its development with great care, the
thyroid body arises on the third day as an, involution from the hypoblast of the
throat opposite the point of origin of the second arterial arch. This involution
becomes by the fourth day a solid mass of cells, and by the fifth ceases to be
connected with the epithelium of the throat, becoming at the same time bilobed.
By the seventh day it has travelled somewhat backwards, and the two lobes
have completely separated from each other. By the ninth day the whole is
invested by a capsule of connective tissue, which sends in septa dividing it into
a number of lobes or solid masses of cells, and by the sixteenth day it is a paired
body composed of a number of follicles, each with a ' membrana propria,' and
separated from each other by septa of connective tissue, much as in the adult.
22. Coincidently with the appearance of these several
rudiments of important organs in the more or less modified
splanchnopleure-folds, the solid trunk of the embryo is
undergoing marked changes.
When we compare a transverse section taken through
say the middle of the trunk at the end of the third day
(Fig. 44), with a similar one of the second day (Fig. 20), or
even the commencement of the third day (Fig. 41), we
are struck with the great increase of depth (from dorsal
to ventral surface) in proportion to breadth. This is partly
due to the slope of the side walls of the body having become
much steeper as a direct result of the rapidly progressing
folding off of the embryo from the yolk-sac. But it is also
brought about by the great changes both of shape and
structure which are taking place in the protovertebrae as
well as by the development of a mass of tissue between the
notochord and the hypoblast of the alimentary canal.
23. The protovertebra3 on the second day, as seen in a
transverse section, Fig. 20, P.v., are somewhat quadrilateral
in form but broader than high. Each at that time consists
of a somewhat thick cortex of radiating rather granular
columnar cells, enclosing a small kernel of spherical more
transparent cells.
Remak and after him Kolliker have described the centre of the proto-
vertebrsfi as being simply fluid without structural elements. His explicitly
denies this in the case of the protovertebrse of the neck, and it seems probable
that the centre is in all cases really occupied by transparent spherical cells.
Towards the end of the second and the beginning of
the third day, the central cells increase rapidly in number
(Fig. 41), and towards the end of the latter day (Fig. 44), as
it were lift up and push out the columnar cortex above and
at the outer side. In this way the portions forming the
THE MUSCLE-PLATES.
135
SO
SECTION THROUGH THE DORSAL KEGION OP AN EMBRYO AT THE END OF THE
THIRD DAY.
Am. amnion. m. p. muscle-plate. C. F. cardinal vein. Ao. dorsal aorta. The
section passes through the point which the dorsal aorta is just commencing
to divide into two branches. Ck. notochord. W. d. Wolffian duct. W. b.
commencing differentiation of the mesoblast cells to form the Wolffian
body. ep. epiblast. SO. somatopleure. Sp. splanchnopleure. hy. hypoblast.
The section passes through the point where the digestive canal communicates
with the yolk-sac, and is consequently still open below.
This section should be compared with the section through the dorsal region of
an embryo at the commencement of the third day (Fig. 41). The chief differences
between them arise from the great increase in the space (now filled with meso-
blast-cells) between the notochord and the hypoblast. In addition to this we
have in the later section the completely formed amnion, the separation of the
muscle-plate from the protovertebrse, the formation of the Wolffian body, etc.
The mesoblast including the Wolffian body and the muscle-plate (m.p.) is
represented in a purely diagrammatic manner. The amnion, of which only the
inner limb or true amnion is represented in the figure, is seen to be composed of
epiblast and a layer of mesoblast ; though in contact with the body above the top
of the medullary canal, it does not in any way coalesce with it, as might be con-
cluded from the figure.
136 THE THIRD DAY. [CHAP.
upper and outer border of the protovertebra become separated
from the rest of the cortex, the columnar cells of the latter
at the same time losiDg their distinctive characters and
ceasing to be distinguishable from the central cells. As a
consequence of this the whole protovertebra, while thus
increasing in breadth out of proportion to its height, becomes
split up into two portions which lie one above the other.
Of these the upper one, which is from the first the most
flattened and longest, follows the curvature of the body-wall
and thus from being nearly horizontal comes to slope at a
considerable angle. It now receives the name of muscle-plate,
Fig. 44, m.p. Of its subsequent changes we shall have to speak
in a succeeding chapter.
The remaining portion of the original protovertebra is
still called protovertebra and begins to extend inwards over
the neural canal above and towards the notochord below.
24. Meanwhile the breadth or rather depth of the trunk
is also being increased by the development of mesoblastic
cells between the notochord and hypoblast.
In a transverse section of a 45 hours' embryo a consider-
able mass of cells may be seen collected between the pro-
tovertebra and the point where the divergence into somato-
pleure and splanchnopleure begins (Fig. 20 just below W.d.,
also Fig. 41, the diagrammatically shaded part lying between
p.v. and g.e). This mass of cells, which we may speak of as
the intermediate cell-mass, now passes without any very sharp
line of demarcation into the protovertebra itself ; and as the
folding in of the side wall progresses, increases in size and
grows in between the notochord and the hypoblast, but does
not accumulate to a sufficient extent to separate them
widely until the end of the third or beginning of the fourth
day.
The fusion between the intermediate cell-mass and the
outer and under portions of the altered pro to vertebras
becomes so complete on the third day that it is almost
impossible to say which of the cells in the immediate
neighbourhood of the notochord are derived from the proto-
vertebrse and which from the intermediate cell-mass. It
seems probable however that the cells which form the
immediate investment of the notochord really belong to the
protovertebraB.
V.] THE CRANIAL NERVES. 137
Schenk (Wien. Sitz. Bericlit. 1868) describes all the cells which invest
the hypo-blast of the digestive tract, as primarily derived from the proto-
vertebrse, with the exception of the peritoneal epithelium, which alone, he con-
siders, is the representative of the original mesoblast of the splanchnopleure.
According to this view, the muscles of the walls of the alimentary canal, and the
'hypaxial' muscles, are derived from the original protovertebrse, quite as much
as those muscles which spring out of the muscle-plate. In the absence of any
satisfactory means of distinguishing the cells of the intermediate cell-mass
from those of the protovertebree, this view must be considered as at least very
doubtful.
25. In the mesoblast, which lies by the side of the
hind brain, and which though not divided into protover-
tebrse is the prolongation forwards of the same column of
mesoblast out of which in the trunk the protovertebrse are
formed, there appear on either side in the course of the third
day a series of four small opaque masses, somewhat pearshaped
with the stalk directed away from the middle line. These
are the rudiments of four cranial nerves, of which two lie in
front of and two behind the auditory vesicle.
The most anterior of these is the rudiment of the fifth
nerve (Figs. 25, V. 45, V). Its narrowed outer portion or
HEAD OF AN EMBRYO CHICK OF THE THIRD DAY (seventy-five hours) VIEWED
SIDEWAYS AS A TRANSPARENT OBJECT (from Huxley).
la. cerebral hemispheres. 16. vesicle of the third ventricle. II. mid-brain.
III. hind-brain, g. nasal pit. a. optic vesicle. &. otic vesicle, d. infundi-
bulum. e. pineal body. h. notochord. V. fifth nerve. VII. seventh nerve.
VIII. united glossopharyngeal and pneumogastric nerves, i, 2,^3, 4, 5 the
five visceral folds.
The stage here represented is a little later than that shewn in Fig. 25, with
which it should be compared.
138 THE THIRD DAY. [CHAP.
stalk divides into two bands or nerves. Of these one passing
towards the eye terminates at present in the immediate
neighbourhood of that organ. Compare Fig. 46. The other
branch (the rudiment of the inferior maxillary branch of the
fifth nerve) is distributed to the first visceral arch (Fig. 46).
The second mass (Fig. 25, VII. 45, VII.) is the rudiment
of the seventh, or facial nerve. It is the nerve of the second
visceral arch.
The two masses behind the auditory vesicle represent
the glossopharyngeal and pneumogastric nerves (Fig. 45, VIII.,
Fig. 46, G. Ph. and Pg.}. At first united, they subsequently
become separate. The glossopharyngeal supplies the third
arch, and the pneumogastric the fourth arch.
These four masses, representing four important mixed cranial nerves,
seem to be derived directly from the mesoblast surrounding the hind-brain.
It is worthy of notice that they are mixed, sensory and motor, nerves; for,
restricted as are the sensory functions of the seventh, and the motor func-
tions of the pneum.ogastric in the adult mammal, the study of their com-
parative physiology leaves no doubt as to the essentially mixed nature of each.
It is also worthy of note that of the third, fourth and sixth nerves, no such early
rudiments appear ; and there are reasons for thinking that these are in reality
intercranial branches, the third and fourth of the fifth, and the sixth of the
seventh nerves. The purely sensory nerve or rather sense-nerve, the auditory,
seems to have a different origin altogether from all the above, though it may
perhaps be looked upon as the dorsal branch of the seventh, while the erratic
hypoglossal appears to be distinctly a spinal nerve.
Of the interesting relations of these cranial nerves to the visceral arches, we
shall have to speak more fully in the second part of this work, when we describe
the more primitive forms in the lower vertebrata.
At the same time that these ganglia make their appearance, or a little earlier,
near the beginning of the third or end of the second day, there may be seen in
the region of the hind-brain, lines which appear to divide off the mesoblast on
either side into masses somewhat resembling protovertebree. Of these masses
there are four or five on each side, generally three in front of. and two behind
the optic vesicle. They were first noticed by Remak, and are easily dis-
tinguished from the rudiments of the cranial nerves. They at first sight suggest
the idea of an initial and transitory segmentation of the cranial mesoblast into
protovertebrse. It seems possible that they are, in reality, appearances pro-
duced by a series of very characteristic transverse wrinkles into which the walls
of the hind-brain are at this time thrown, arid which subsequently disap-
pearing altogether, as the walls increase in thickness, may perhaps be viewed
as indications of an aborted segmentation of the hind-brain into a series of
vesicles. The true nature of these quadrate masses is still very problematical.
26. On the second day the newly formed Wolffian duct
extended along the greater part of the length of the embryo
as a tube resting on the mass of cells which we have already
called the intermediate cell-mass.
V.] THE WOLFFIAN DUCT. 139
On the third day, in consequence of the continually
folding in of the somatopleure and especially of the splanch-
nopleure, as well as owing to the changes taking place in the
protovertebrao, the Wolffian duct undergoes a remarkable
change of position. Instead of lying as on the second day
immediately under the epiblast (Fig. 20, W.d.), it is soon
found to have apparently descended into the middle of the
intermediate cell-mass (Fig. 41, w.d.) and at the end of
the third day occupies a still lower position and even pro-
jects somewhat into the pleuroperitoneal cavity. (Fig. 44,
W.d.)
Towards the end of the day the rudiments of the Wolffian
bodies (Fig. 44, W.b.) begin to make their appearance in con-
nection with the ducts, but the consideration of these may
conveniently be reserved to the next chapter.
27. The chief events then which take place on the
third day are as follows :
1. The turning over of the embryo so that it now
lies on its left side.
2. The cranial flexure round the anterior extremity of
the notochord.
3. The completion of the circulation of the yolk-sac ;
the increased curvature of the heart, and the demarcation of
its several parts ; the appearance of new aortic arches, and
of the cardinal veins.
4. The formation of four visceral clefts and five visceral
arches.
5. The involution to form the lens, and the formation
of the secondary optic vesicle.
6. The closing in of the otic vesicle.
7. The formation of the nasal pits.
8. The appearance of the vesicles of the cerebral hemi-
spheres ; the separation of hind-brain into cerebellum and
medulla oblongata.
9. The completion of the fore-gut and of the hind-gut ;
the division of the former into oesophagus, stomach and
duodenum, of the latter into large intestine and cloaca.
10. The formation of the lungs as two diverticula from
the alimentary canal immediately in front of the stomach.
11. The formation of the liver and pancreas: the former
as two diverticula from the duodenum, which subsequently
140 THE THIRD DAY. [CHAP. V.
become united by solid outgrowths ; the latter as a single
diverticulum also from the duodenum.
12. The changes in the proto vertebrae and the appear-
ance of the muscle-plates.
13. The appearance of the cranial nerves in the meso-
blast adjoining the hind brain.
14. The change in position of the Wolffian duct.
CHAPTER VI.
THE CHANGES WHICH TAKE PLACE DURING THE FOURTH
DAY.
1. ON opening an egg in the middle or towards the end
of the fourth day, a number of points in which progress has
been made since the third day are at once apparent. In the
first place, the general growth of the embryo has been very
rapid, so that its size is very much greater than on the
previous day. In the second place, the white of the egg has
still further diminished, the embryo lying almost in immediate
contact with the shell membrane.
The germinal membrane embraces more than half the
yolk, and the vascular area is about as large as a halfpenny.
Corresponding to the increased size of the embryo, there
is a great increase in the quantity of blood circulating in the
vascular area as a whole, though the sinus terminalis is
already less distinct 'than it was.
2. The amnion becomes increasingly conspicuous. It is
now seen as a distinct covering obscuring to a certain extent
the view of the body of the chick beneath, and all traces of
the junction of its folds are by this time lost. As yet there
is very little fluid in the amniotic sac proper, so that the
true amnion lies close upon the embryo.
3. The folding off of the embryo from the yolk sac has
made great progress. The splanchnic stalk, which on the
third day was still tolerably wide, inasmuch as about one
third of the total length of the alimentary canal was as yet
quite open to the yolk sac below, now becomes so much con-
stricted by the progressive closing in of the splanchnopleure
142
THE FOURTH DAY.
[CHAP.
FIG. 46.
vir *^
*. s ;\ *rlr?* c *
M.P
EMBRYO AT THE END OF THE FOURTH DAY SEEN AS A TRANSPARENT OBJECT.
The amnion has been completely removed, the cut end of the somatic stalk
is shewn at S.S. with the allantois (Al.) protruding from it.
C.H. cerebral hemisphere. F. B. fore brain or vesicle of the third ventricle
with the pineal gland (Pn.) projecting from its summit. M.B. mid brain.
Cb. cerebellum. IV. V. fourth ventricle. L. lens. ch.s. choroid slit.
Owing . to the growth of the optic cup the two layers of which it is
composed cannot any longer be seen from the surface, but the posterior
surface of the choroid layer alone is visible. Cen. V. auditory vesicle.
s.m. superior maxillary process. ijP. iF. etc. first, second, third and
fourth visceral folds. V. fifth nerve sending one branch to the eye, the
ophthalmic branch, and another to the first visceral arch. VII. seventh
nerve passing to the second visceral arch. G. Ph. glossopharyngeal nerve
passing towards the third visceral arch. Pg. pneumogastric nerve passing
towards the fourth visceral arch. iv. investing mass. No attempt has
been made in the figure to indicate the position of the dorsal wall of
the throat, which cannot be easily made out in the living embryo.
ch. notochord. The front end of this cannot be seen in the living embryo.
It does not end however as shewn in the figure, but takes a sudden bend
downwards and then terminates in a point. Ht. heart seen through the
walls of the chest. M. P. muscle-plates. W. wing. H. L. hind limb.
Beneath the hind limb is seen the curved tail.
VI.] THE LIMBS. 143
folds, that the alimentary canal maybe said to be connected
with the yolk sac by a very narrow neck only. This rem-
nant of the splanchnic stalk we may now call the umbilical
duct; though narrow, it is as yet quite open, affording
still free communication between the inside of the yolk sac
and the interior of the alimentary canal.
The somatic stalk, though narrowing somewhat, is much
wider than the splanchnic stalk, so that a considerable
ring-shaped space exists between the two.
4. Another very prominent feature is the increase in the
cranial flexure. During the third day, the axis of the front
part of the head was about at right angles to the long axis of
the body ; the whole embryo being still somewhat retort-
shaped. On this day, however, the flexure has so much
increased that the angle between the long axis of the body
and that of the front segment of the head is an acute one,
and the mouth is turned so as completely to face the chest.
The tail-fold, which commenced to be noticeable during
the third day, has during this day increased very much, and
the somewhat curved tail (Fig. 46) forms quite a conspicuous
feature of the embryo. The general curvature of the body has
also gone on increasing, and as the result of these various
flexures, the embryo has very much the appearance of being
curled up on itself (Fig. 46).
5. The distinct appearance of the limbs must be reckoned
as one of the most important events of the fourth day.
Owing to the continued greater increase of depth than of
breadth, the body of the embryo appears in section (Fig. 47)
higher and relatively narrower than even on the third day, and
the muscle-plates, instead of simply slanting downwards, come
to be nearly vertical in position. Not far from the line
which marks their lower ends, the somatopleure, almost
immediately after it diverges from the splanchnopleure, is
raised up (Fig. 47, W.R.} into a low rounded ridge which runs
along nearly the whole length of the embryo from the neck
to the tail.
It is on this ridge, which is known as the Wolffian ridge,
tha,t the limbs first appear as flattened conical buds project-
ing outwards. They seem to be local developments of the
ridge, the rest of which becomes less and less prominent as
they increase in size. Each bud, roughly triangular in sec-
144
THE FOUKTH DAY.
FIG. 47.
[CHAP.
nc.
SECTION THROUGH THE LUMBAR KEGION or AN EMBRYO AT THE END OF THE
FOURTH DAT. , .
n. c. neural canal, p. r. posterior root of spinal nerve with ganglion, a. r. anterior
root of spinal nerve. A. 6r. C. anterior grey column of spinal cord. A. W. C.
anterior white column of spinal cord just commencing to be formed, and
not very distinctly marked in the figure, m. p. muscle plate, ch. notochord.
W. R. Wolffian ridge. A. 0. dorsal aorta. V. c. a. posterior cardinal vein.
W. d. Wolffian duct; its section is not circular, owing to its being cut
through at a point where it is being joined by one of the tubules. W. b.
Wolffian body, consisting of tubules and Malpighian corpuscles. One of
the latter is represented on each side, that on the left hand having its
glomerulus entirely filled with blood -corpuscles, g. e. germinal epithelium.
M. d. commencing involution of germinal epithelium to form the duct of
Muller. d. alimentary canal. M . commencing mesentery. S. 0. somato-
pleure. S. P. splanchnopleure. V. blood-vessels, pp. pleuroperitoneal
cavity.
VI,] THE NASAL PITS. 145
tion, consists of somewhat dense mesoblast covered by epiblast
which on the summit is thickened into a sort of cap. The
front limbs or wings (Fig. 46) arise just behind the level of
the heart, and the hind limbs in the immediate vicinity
of the tail. The first traces of them can be seen towards
the end of the third, but they do not become conspicuous
till the fourth day, by the end of which the two pairs may
be already distinguished by their different shapes. The front
limbs are the narrowest and longest, the hind ones being
comparatively short and broad. Both are flattened from
above downwards and become more so as their growth
continues.
6. In the head, the vesicles of the cerebral hemispheres
are rapidly increasing in size, overlapping the insignificant
olfactory vesicles in front, and encroaching on the 'tween-
brain or vesicle of the third ventricle behind. The mid-brain
is now, relatively to the other parts of the brain, larger than
at any pther epoch, and an indistinct median furrow on its
upper surface indicates its division into two lateral halves.
The great increase of the mesoblastic contents of the second-
ary optic vesicle or involuted retinal cup causes the two eye-
balls to project largely from the sides of the head (Fig. 48, Op).
The mass of mesoblast which invests all the various parts of the
brain, is not only growing rapidly below and at the sides, but
is also undergoing developments which result in the forma-
tion of the primitive skull, and of which we shall speak in
detail in a subsequent separate chapter. All these events,
added to the cranial flexure spoken of above, give to the
anterior extremity of the embryo a shape which it becomes
more arid more easy to recognize as that of a head.
7. Meanwhile the face is also being changed. The two
nasal pits were on the third day shallow depressions with
thickened borders complete all round. As the pits deepen on
the fourth day by the growth upwards of their rims, a break
is observed in each rirn in the form of a groove (Fig. 48, N)
directed obliquely downwards towards the cavity of the mouth.
The fronto-nasal process or median ridge (Fig. 48, nf), which
on the third day rose up between the superficial projections
caused by the bulging anterior extremities of the vesicles of
the cerebral hemispheres, and on the fourth day becomes
increasingly prominent, separates the two grooves from each
146 THE FOUETH DAY. [CHAP.
FIG. 48.
A. HEAD OF AN EMBRYO CHTCK OF THE FOURTH DAT VIEWED FROM BELOW
AS AN OPAQUE OBJECT. (Chromic acid preparation).
CH. cerebral hemispheres. FB. vesicle of the third ventricle. Op. eyeball.
nf. naso-frontal process. M. cavity of mouth. S. M. superior maxillary
process of .P. i, the first visceral fold (inferior maxillary process). F. 2, F. 3.
second and third visceral folds. N. nasal pit.
In order to gain the view here given the neck was cut across between the
third and fourth visceral iolds. In the section e thus made, are seen the
alimentary canal al with its collapsed walls, the neural canal n.c., the notochord
ch., the dorsal aorta AO., and the vertebral veins V.
The incision has been carried just below the upper limit of the pleuroperi-
toneal cavity, consequently a portion of the somatopleure appears at the angle
between the two third visceral folds. Almost embraced by the piece of somato-
pleure is seen the end of the bulbus arteriosus Ao.
In the drawing the nasal groove has been rather exaggerated in its upper
part. On the other hand the lower part of the groove where it runs between
the superior maxillary process S.M. and the broad naso-frontal process was, in
this particular embryo, extremely shallow and indeed hardly visible. Hence
the end of the superior maxillary process seems to join the inner and not, as
described in the text, the outer margin of the nasal groove. A few hours later
the separation of the two would have been very visible.
B. The same seen sideways, to shew the visceral folds. Letters as before.
other, and helps to form the inner wall of each of them.
Abutting on the outer side of each groove and so helping to
form the outer wall of each, lie the ends of the superior
maxillary processes of the first visceral arch (Fig. 48 B, sm),
which like the fronto-nasal process are increasing in size. By
their continued growth, the groove is more and more deepened,
and leading as it does from the nasal pit to the cavity of the
mouth, may already be recognized as the rudiment of the
passage of the posterior nares.
VI.] THE CRANIAL NERVES. 147
8. Daring the latter half of the fourth day there appears
at the bottom of the deep lozenge-shaped cavity of the mouth,
in the now thin wall dividing it from the alimentary canal, a
longitudinal, or according to Gotte a vertical slit which, soon
becoming a wide opening, places the two cavities in complete
communication.
The cavity of the mouth, being, it will be remembered,
formed partly by depression, partly by th,e growth of the sur-
rounding folds, is lined entirely with epiblast, from which the
epithelium of its surface and of its various glands is derived.
In this respect, as Remak pointed out, it differs from all the
rest of the alimentary canal, w^hose whole epithelium is formed
out of hypoblast.
9. By the side of the hind-brain, in which the cerebellum,
through the increased thickening of its upper walls, is be-
coming more and more distinct from the medulla oblongata,
both in front and behind the auditory vesicle, in which the
rudiments of the cochlea and recessus vestibuli are already
visible, the cranial ganglia and nerves are acquiring increased
distinctness and size. They may be very plainly seen when
the head of the fresh embryo is subjected to pressure.
The foremost is the fifth cranial nerve (Fig. 46, V.) with
its Gasserian ganglion ; it lies a little way behind the anterior
extremity of the notochord immediately below the cerebellum.
Next to this comes the seventh (Fig. 46, VII.} nerve, starting
just in front of the ear-vesicle, and extending far downwards
towards the second visceral arch. The two nerves which lie
behind the ear- vesicle are now obviously separate from each
other; the front one is the glossopharyngeal (Fig. 46, G. Ph.],
and the hinder one already shews itself to be the pneumo-
gastric (Fig. 46> P#.)-
10. Besides the progressive changes of the alimentary
canal and its surroundings, which we incidentally described
in the last chapter, and the closure of the mid-gut to form the
umbilical duct, of which we have also already spoken, a totally
new and most important appendage of the digestive tract,
the allantois, becomes for the first time conspicuous on this
day, though the first rudiments of it appeared on the third.
Soon after its appearance the allantois may easily be recog-
nized as a pear-shaped vesicle lying in the hinder district of
the pleuroperitoneal space, and connected with the under
148 THE FOURTH DAY. [CHAP.
surface of the cloaca by a long hollow stalk, which places its
cavity in communication with that of the alimentary canal.
Both vesicle and stalk have an outer coat of mesoblast and
an inner lining of what apparently is hypoblast. So much
any observer may readily determine for himself; but of the
earliest stages of the development of this organ different
embryologists have given very different accounts.
Von Baer believed that, soon after the cloaca was formed by the enlargement
of the caecal hind end of the alimentary canal, the allantois arose from it as
a spherical diverticulum, generally visible about the middle of the third day, in
whose formation both of the coats of the alimentary canal took part. This
spherical diverticulum gradually lengthened out into a pear-shaped vesicle,
connected with the cloaca by a hollow stalk which rapidly narrowed and
lengthened, until the allantois formed an independent hollow body, composed
of an outer coat of mesoblast and a lining of hypoblast, and communicating
with the cloaca by a narrow tube of the same construction.
Keichert (Entwicklungsgeschichte, s. 186) on the other hand stated that the
allantois was formed of two solid outgrowths from the mesoblast of the somato-
pleure, which subsequently coalesced and became hollow; but believed that it
was primarily connected with the Wolffian ducts and not with the cloaca.
According to Remak (Entwicklung, 57, 58) it is formed by two solid
vascular outgrowths of the mesoblast of the body-wall, one on each side of the
middle line, which project in the pleuroperitoneal cavity near to the cloaca.
These two outgrowths coalesce, and then grow up, till they come in contact with
FIG. 49.
LONGITUDINAL SECTION OF THE TAIL-END OP AN EMBRYO CHICK AT THE
COMMENCEMENT OF THE THIRD DAY (Dobrjnin).
t. the tail, m. the axial mesoblast of the body, about to form the protovertebras.
x f . the roof of x". the neural canal. Dd. the hind end of the hind-gut. SO.
somatopleure. Spl. splanchnopleure. u. the mesoblast of the splancbno-
pleure carrying the vessels of the yolk-sac, pp. pleuroperitoneal cavity.
Df. the epithelium lining the pleuroperitoneal cavity. All. the commencing
allantois. w. and y. the hypoblast thickened and projecting on either side of
the opening of the allantois.
VL]
THE ALLANTOIS. 149
the wall of the cloaca: with this they unite, and form together a solid
spherical body, bearing on its external surface a median furrow, indicating its
double origin. A narrow diverticulum of hypoblast now passes into the mass,
and forms within it a cavity, which is at first small and, corresponding to the ex-
ternal contour of the body, to a certain extent double. The hypoblast diverticulum
grows rapidly, while its mesoblastic covering remains nearly stationary, so that
the mesoblast finally comes to form a thin coating only over the hypoblast.
His (op. cit. ) gives a somewhat elaborate and complicated account of the
development of the allantois; which is accepted by Waldeyer (Eierstock und
Ei) and Bornhaupt (Untersuckuny iiber die Entwickelung des Urino-genitalsy stems
beim Hiihnchen, Riga, 1867).
It appears to be nearly the same as the fuller account given by Dobrynin
(Ueber die erste Anlage der Allantois. Sitz. der k. Akad. Wien, Bd. 64, 1871),
of which the following is an abstract.
When the first commencement of the hind fold takes place, immediately
beyond the point where the hypoblast turns back to assume its normal direction
over the yolk-sac, a narrow diverticulum which points backwards and some-
what upwards is formed by a special flexure of the splanchnopleure. The open
end of the diverticulum, Fig. 49, All. f looks forwards towards the wide opening
connecting the digestive tract with the yolk-sac; its blind end points directly
towards the pleuroperitoneal cavity. This diverticulum is the commencing
allantois. It is lined by hypoblast, while its exterior is composed of the mesoblast
of the splanchnopleure.
As the folding in to form the digestive tract increases, the diverticulum alters
FIG. 50*
LONGITUDINAL SECTION OP THE TAIL-END OF AN EMBRYO CHICK AT THE
MIDDLE OF THE THIRD DAY (Dobrynin).
t. the tail; the line of reference points to the axial mesoblast at the tail.
x. epiblast. SO. somatopleure. ra. mesoblast to form the body wall.
V. commencing amniotic fold. Hp. space between the true and the false
amnion. pp. Pleuroperitoneal cavity. Spl. splanchnopleure. G. Cloacal
enlargement of the alimentary canal. Dd. dorsal wall of the alimentary
canal. All. vesicle of the allantois having a wide opening into the alimen-
tary canal.
150 THE FOURTH DAY. [CHAP.
its position and becomes quite parallel with the commencing digestive tract.
Its cavity is separated from that of the digestive canal by a projection of
mesoblast covered by hypoblast ; but both open freely in front into the common
splanchnic stalk.
In the next stage it still further alters its position, and forms, Fig. 50, a rather
wide vesicle lying immediately below the hind end of the digestive canal, with
which it communicates freely by a still broad opening ; its blind end projects
freely into the pleuroperitoneal cavity below. It was in this condition when
Von Baer first observed it.
At the time when these changes are taking place, the somatopleure is
being folded in to form the walls of the body; and as the folds, one on either
side, are carried forward from the extreme end of the tail, they present them-
selves, when seen from within or in sections, as two ridges projecting towards
the sides of the allantois. Reaching the allantois these ridges fuse with its wall,
and in this way reduce the pleuroperitoneal cavity immediately below the allan-
tois to quite a narrow space, which is seen in section as a mere chink. Remak
apparently mistook these infoldings of the somatopleure, and the consequent
projections into the pleuroperitoneal cavity, for the first formation of the
allantois, although they have in fact little or no connection with it.
We may therefore probably consider the , allantois as a
portion of the cloaca, which grows forward and becomes an
independent spherical vesicle, still however remaining con-
nected with the cloaca by a narrow canal which forms its neck
or stalk. The opening of the allantois into the cloaca is on
the under side of the latter. Both the neck and vesicle of
the allantois are lined by hypoblast, while its exterior is com-
posed of the mesoblast of the splanchnopleure. From the
first the allantois lies in the pleuroperitoneal cavity. In this
cavity it grows forwards till it reaches the front limit of the
hind-gut, where the splanchnopleure turns back to reach
the yolk-sac. It does not during the third day project be-
yond this point ; but on the fourth day begins to pass out
beyond the body of the chick along the as yet wide space
between the splanchnic and somatic stalks of the embryo
on its way to that space between the external and internal
folds of the amnion, which it will be remembered is directly
continuous with the pleuroperitoneal cavity (Fig 8, K). In
this space it eventually spreads out over the whole body of
the chick. On the first half of the day the vesicle is still
very small, and its growth is not very rapid. Its mesoblast
wall still remains very thick. In the later half of the day its
growth becomes very rapid and it forms a very conspicuous
object in a chick of that date (Fig. 46, Al.}. At the same
time its blood-vessels become important. To these we shall
presently return.
VI.] THE PROTOVEBTEBILE. 151
11. The proto vertebrae, which by the continued differ-
entiation of the axial mesoblast at the tail end of the embryo
have increased in number from thirty to forty, undergo during
this day changes of great importance. Since these changes
are intimately connected with the subsequent development
of the vertebral column, it will perhaps be more convenient
to describe briefly here the whole series of events through
which the protovertebrse become converted into the per-
manent structures to which they give rise, though many of
the changes do not take place till a much later date than
the fourth day.
The separation of the muscle-plates (Chap. v. 23) left
the remainder of each proto vertebra as a somewhat tri-
angular mass lying between the neural canal and notochord
on the inside, and the muscle-plate and intermediate cell-mass
on the outside (Fig. 44). Already on the third day the upper
angle of this triangle grows upwards, between its muscle-
plate and the neural canal, and meeting its fellow in the
middle line above, forms a roof of mesoblast over the
neural canal, between it and the superficial epiblast. At
about the same time, the inner and lower angle of the
triangle grows inwards towards the notochord, and passing
both below it (between it and the aorta) and above it (be-
tween it and the neural canal), meets a similar growth from
its fellow proto vertebra of the other side, and thus com-
pletely invests the notochord with a coat of mesoblast,
which, as seen in Fig. 47, is at first much thicker on the
under than on the upper side.
While the inner portion of each proto vertebra is thus
extending inwards around both notochord and neural canal,
the remaining outer portion is undergoing a remarkable
change. It becomes divided into an anterior or praeaxial,
and a posterior or postaxial segment. The anterior, which is
the larger and more transparent of the two, is the rudiment
of the spinal ganglion and nerve, while the posterior, which
remains more particularly connected with the extensions
round the neural canal and notochord, goes to form part of
the permanent vertebra.
In this way, each protovertebra, having given rise to a
muscle-plate, is further divided into a ganglionic rudiment,
and into a mass which we may speak of as a "primary"
152 THE FOURTH DAY. [CHAP.
vertebra, consisting as it does of a body or mass investing the
notochord, from which springs an arch covering in the neural
canal.
Both body and arch consist at this epoch of but slightly
differentiated mesoblast, and the arch springs, to a certain
extent, not oily from the posterior segment of the protover-
tebra, but also from the anterior or ganglionic segment :
though, as seen in Fig. 47, it is far less conspicuous at the
level of the latter than of the former. Both neural canal
and notochord are thus furnished from neck to tail with a
complete investment of mesoblast, still marked, however, by
the transparent lines indicating the fore and aft limits of
the several protovertebrse. This is sometimes spoken of as
the " membranous " vertebral column.
The ganglionic rudiment, placed anteriorly to its corre-
sponding primary vertebra, consists in chief of a large oval
swelling, the ganglion of the posterior root (Fig. 47, pr).
At a little distance beyond its ganglion, the posterior root is
joined by the anterior root (ar) ; and the two form together
the common nerve-trunk, which is at first very short. Com-
pared with either root or with the nerve-trunk the ganglion,
at this epoch and for some time afterwards, seems dispropor-
tionably large. At first, neither root is connected with the
involuted epiblast of the neural canal. Very speedily, how-
ever, they both come to be united with that portion of the
neural tube which, as we shall presently state, gives origin
to the grey matter of the spinal cord. It is, however, easier
to trace the fibres of the anterior root into the cord, than
those of the posterior, and they can be followed in it for a
greater distance.
On the fourth day the nerves are composed of cells whose protoplasm is
beginning to become converted into fibres. Amongst these fibres, the nuclei
of the cells with distinct nucleoli are thickly scattered. On the sixth day and
still more on the seventh the fibrillated structure of the nerves is much more
distinct and the nuclei are far less numerous.
The ganglia on the fourth day are composed of numerous nuclei surrounded
by protoplasm, between which the fibres of the nerves pass. Covering this mass
of nucleated cells is a layer of mesoblast (also derived from the tissue of the
protovertebra) which, by the sixth day, forms a kind of sheath around them.
The cells of the ganglia from the fourth to the sixth day contain round
granular nuclei with distinct nucleoli very similar to the nuclei of the
ordinary mesoblast-cells. The limits of the protoplasm of the individual cells
are as a general rule not easily seen, but with care may be made out. The
amount of protoplasm round each nucleus appears however to be very small.
VI.] THE SPINAL GANGLIA. 153
The fibres of the nerve can easily be traced through the ganglion. In
section they appear to have a somewhac wavy course, and by interlacing divide
the ganglion into a number of elongated areas in each of which is a row of
nuclei. In sections of the sixth day it is not possible to trace a connection
between the nerve-fibres and the cells. The nuclei are most numerous at
the lower ends of the ganglia.
On the seventh day, the nuclei have become larger, and where the outline
of a cell can be distinctly seen it is generally somewhat angular. In sections
it is still on the seventh day not possible to trace any connection between the
cells and the nerve-fibres.
Remak (op. cit.) speaks of the ganglia being composed of non-nucleated
spheres, and Lockhart Clarke (Philosopkical Transactions, 1862) also describes
the ganglia-cells as "cells or nuclei" which are at first mere rounded masses
of protoplasm, and do not acquire a nucleus till a later period. Both of these
statements are according to our observations incorrect.
In the later stages according to Lockhart Clarke's account (loc. cit.) the cells
of the ganglia send out processes which anastomose together into a fine
network. The ceils also become connected with the nerve fibres, which can
sometimes be seen to divide in the ganglion into a fine brush-like bundle of
fibrillae. At this time the cells possess a distinct nucleus and nucleolus.
These changes he describes aa completed by the ninth day of incubation.
His believes that the spinal nerves are derived from downward prolonga-
tions of the superficial epiblast descending between the pro to vertebrae. This
view has not been corroborated by subsequent observers.
12. The remaining portions of the protovertebrae form-
ing the primary vertebrae or membranous vertebral column
spoken of in the last paragraph, are converted into the per-
manent vertebras ; but their conversion is complicated by a
remarkable new or secondary segmentation of the whole
vertebral column.
On the fourth day, the transparent lines marking the
fore and aft limits of the proto vertebras are still distinctly
visible. On the fifth day, however, they disappear, so that
the whole vertebral column becomes fused into a homoge-
neous mass whose division into vertebrae is only indicated by
the series of ganglia. This fusion, which does not extend to
the muscle-plates in which the primary lines of division still
remain visible, is quickly followed by a fresh segmentation,
the resulting segments being the rudiments of the permanent
vertebras. The new segmentation, however, does not follow
the lines of the earlier division, but passes between the
ganglionic and the vertebral portions, in fact, through the
middle, of each protovertebra. In consequence, each spinal
ganglion and nerve ceases to form the front portion of the
primary vertebra, formed out of same protovertebra as itself,
but is attached to the hind part of the permanent vertebra
154 THE FOURTH DAY. [CHAP.
immediately preceding. Similarly, the rudiment of each
vertebral arch covering in the neural tube no longer springs
from the hind part of the protovertebra from which it is an
outgrowth, but forms the front part of the permanent ver-
tebra, to which it henceforward belongs. The ganglia are still,
however, the most conspicuous portions of each segment.
By these changes this remarkable result is brought about,
that each permanent vertebra is formed out of portions of
two consecutive proto vertebrae. Thus, for instance, the tenth
permanent vertebra is formed out of the hind portion of the
tenth protovertebra, and the front portion of the eleventh
protovertebra, while its arch, now attached to its front
part, was attached to the hind part of the tenth protovertebra.
The new segmentation is associated with or rather is
caused by histological changes. At the time when the fusion
takes place, the mesoblast, which in the form of processes
from the proto vertebral bodies surrounds and invests the
notochord, has not only increased in mass but also has
become cartilaginous, so that, as Gegenbaur (Untersuchung
zur vergleichenden Anatomic der Wirbelsdule bei Amphibien
und Reptilien, Leipzig, 18G2) points out, we have for a short
period on the fifth day a continuous and unsegmented carti-
laginous investment of the notochord.
This cartilaginous tube does not however long remain uni-
form. At a series of points corresponding in number to the
original proto vertebrae it becomes connected with a number
of cartilaginous arches which appear in the protovertebral
investment of the neural canal. These arches, which thus
roof in the neural canal and each of which arises opposite to
the vertebral portion of each protovertebral body, are the
cartilaginous precursors of the osseous vertebral arches. We
further find that the portions of the cartilaginous tube from
which the arches spring come to differ histologically from the
portions between them not connected with arches : they are
clearer and their cells are less closely packed. There is
however at this period no distinct segmentation of the
cartilaginous tube, but merely a want of uniformity in its
composition.
The clearer portions, from which the arches spring, form
the bodies of the vertebrce, the segments between them the
intervertebral regions of the column.
VI.] SECONDARY SEGMENTATION OF VERTEBRAL COLUMN. 155
W. Schwarck (Beitrdge zur Entwicldung der Wirbelstiule ~bd den Vogeln.
Anatomische Studien, Dr JJasse, in. Heft, 1872) states that both in the
intervertebral and the vertebral segments the cartilage is divided into two
layers, an inner, central, and an outer peripheral. This division is less marked
in the intervertebral than in the vertebral region.
The inner layer in the vertebral region he speaks of as "the body of the
vertebra belonging to the notochord," and the external layer as "the skeleton-
forming layer."
On the fifth day a division takes place of each of the in-
tervertebral segments into two unequal parts ; a larger one
appertaining to the vertebra in front and a smaller one to. the
vertebra behind. To the larger segment the spinal ganglia
naturally remain attached, and thus comes about the altera-
tion of their place in relation to the vertebrae which we before
spoke of.
This fresh segmentation is not well marked, if indeed it
takes place at all in the sacral region.
Each arch at its first appearance corresponds to about the
middle of a vertebral portion of a proto vertebra, but after the
secondary segmentation the portion of each vertebra behind
its arch grows more quickly than that in front, and thus after
a while the arches seem to spring from the front rather than
from the middle of the vertebral segments.
To recapitulate: the original protovertebras lying side by
side along the notochord, after giving off the muscle-plates, and
dividing lengthways into ganglionic and vertebral portions,
grow around, and by fusing together completely invest,
with mesoblast of protovertebral origin, both neural canal
and notochord.
This investment, of which by reason of its greater growth
the original bodies of the protovertebrse seem to be only an out-
lying part, becomes cartilaginous in such a way that while the
notochord becomes surrounded with a thick tube of cartilage
bearing no signs of segmentation, but having the ganglia
lodged on its exterior at intervals, the neural canal is covered
in with a series of cartilaginous arches, connected to each
other by ordinary mesoblastic tissue only, but passing at their
bases directly into the cartilaginous tube around the noto-
chord.
By a histological process of differentiation the cartila-
ginous tube is divided into vertebral and intervertebral
portions, the vertebral portions corresponding to the arches
156 THE FOURTH DAY* [CHAP.
over the neural canal. Fresh lines of segmentation then
appear in the intervertebral portions, which run in. such a
way that each ganglion is now more closely associated with the
vertebral portion in front of it than with that behind it,
though the latter sprang in part from the same original
protovertebra as itself.
13. Meanwhile from the fourth to the sixth day im-
portant changes take place in the notochord itself.
On its first appearance the notochord was, as we have seen,
composed of somewhat radiately arranged but otherwise
perfectly typical mesoblast-cells.
On the third day some of the central cells become vacuo-
lated, while the peripheral cells are still normal. The vacuo-
lated cells exhibit around the vacuole a peripheral layer of
granular protoplasm in which the nucleus lies embedded,
whilst the vacuoles themselves are filled with a perfectly
clear and transparent material, which in an unaltered con-
dition is probably fluid. Towards the end of the day the
notochord acquires a delicate structureless sheath which is no
doubt a product of its peripheral cells.
According to His there is a cavity in the centre of the notochord on the
third day. We have never observed this, arid . it is denied by Miiller
(Ueber den Bau der Chorda Dorsalis. Jenaische Zeitschrift. Bd. vi. 1871).
On the fourth day all the cells become vacuolated with
the exception of a single layer of flattened cells at the peri-
phery ; and the vacuoles themselves become larger. At the
point where the nucleus lies there is generally rather more
protoplasm than round the remainder of the circumference
of the cells.
On the sixth day all the cells are vacuolated. In each cell
the vacuoles have so much increased at the expense of the
protoplasm that only a very thin layer of the latter is left at
the circumference of the cell, at one part of which, where
there is generally more protoplasm than elsewhere, the
starved remains of a nucleus may generally be detected.
Miiller (loc. cit.) considers that the cells have a membrane. This however
is probably merely a hardened external layer of the protoplasm ; and is stained
by reagents.
Dursy (Zur EntwicldungsgeschicJite des Kopfes des Menscken und der hoheren
Wirbeltkiere) believes that what we have spoken of as vacuoles in the cells are
really intercellular spaces. So that according to his view the notochord is
composed of stellate cells with large round intercellular spaces filled with
transparent intercellular matter. Superficially viewed a section of the noto-
VI.] THE NOTOCHORD. 157
chord of the sixth day might be supposed to have such a structure, but the study
of its development and a careful examination of its structure proves that this is
not a correct account.
According to the measurements of Miiller (loc. cit.} the diameter of the
notochord on the third day is o'op mm. and that of the central cells o'Ois
0*018. On the fourth day the notochord is 0*16 mm. in diameter and its com-
ponent cells are also larger. On the sixth day its diameter is at the maximum,
and reaches 0*2 inm. The central cells measure 0*02 mm.
From these measurements it will be seen that the vacuolation of the cells
of the notochord is accompanied by a rapid growth both in the size of the cells
and in the diameter of the notochord itself.
14. The notochord is on the sixth day at the maximum of
its development, the change which it henceforward under-
goes being of a retrograde character.
From the seventh day onward, it is at various points
encroached upon by its investment. Constrictions are thus
produced which first make their appearance in the interverte-
bral portions of the sacral region. In the cervical region,
according to Gegenbaur, the intervertebral portions are not
constricted till the ninth day, though as early as the seventh
day constrictions are visible in the vertebral portions of
the lower cervical vertebrae. By the ninth and tenth days,
however, all the intervertebral portions have become distinctly
constricted, and at the same time in each vertebral portion
there have also appeared two constrictions giving rise to a
central and to two terminal enlargements. In the space
therefore corresponding to each vertebra and its appropriate
intervertebral portion, there are in all four constrictions and
three enlargements.
On the twelfth day the ossification of the bodies com-
mences. At that time, according to Schwarck (loc. cit.}, the
cartilaginous bodies of the vertebras are composed of an inner
layer in which the cells form lines radiating from the noto-
chord, and an outer layer somewhat sharply separated from
the inner one. In the inner layer, immediately around the
notochord, ossification first commences.
Gegenbaur (loc. cit. p. 67) considers that this layer in which ossification
commences corresponds to the primordial body of the vertebrae in amphibians.
Schwarck is doubtful whether it corresponds to his inner layer of cartilage in
the first stage.
In rare cases ossification first commences as a deposit on the exterior of the
vertebrae.
The first vertebra to ossify is the second or third cervical,
and the ossification gradually extends backwards. It does
158 THE FOUBTH DAY. [CHAP.
not commence in the arches till somewhat later than in the
bodies. For each arch there are two centres of ossification,
one on each side.
We may remind the reader that in the adult bird we find between each of the
vertebrae of a neck and back a cartilaginous disc the meniscus which is
pierced in the centre. These discs are thick at the circumference but thin
off to a fine edge round the central hole. Owing to the shape of these
discs there are left between each pair of vertebrae two cavities, which only
communicate through the central aperture of the meniscus. Through this
central aperture there passes a band connecting the two vertebrae which is called
the 'ligamentum suspensorium..'
In the tail the menisci are replaced by bodies known as the 'annuli fibrosi,'
which precisely resemble the similarly named bodies in mammals. They differ
from the menisci in being attached over their whole surface to the ends of the
vertebral bodies, so that the cavities between the menisci and the vertebrae
cease to exist. They are pierced however by a body corresponding with the
ligamentum suspensorium and knoun as the 'nucleus pulposus.'
In the iritervertebral regions the chorda, soon after the commencement of
ossification, entirely disappears. The cartilage around it however becomes
converted (in the region of the neck) into the ligarnentum suspensorium,
which unites the two vertebrae between which it is placed.
In the tail the cartilage becomes the nucleus pulposus, which corresponds
exactly to the * ligamentum suspensorium' of the neck arid back.
Shortly after the formation of the ligamentum suspensorium the remaining
cartilage of the intervertebral segments is converted in the neck and back
into the meniscus between each two vertebrap, and in the tail into the annulus
fibrosus. Both are absent in the sacrum. These points together with the
anatomy of these parts in the adult were first made out by Jager ( Wirbelkorper-
gelenk der Vogel. Sitz. der k. Akad. Wien, vol. xxxin. 1859).
In the bodies of the vertebrae the notochord does not entirely disappear as
in the intervertebral regions, but, accord ing to Gegenbaur, undergoes ultimately
a direct conversion into cartilage. The contour of the sheath becomes
indistinct ; the cells by the accumulation of matrix round them take on the
form of cartilage-cells, so that at the time of the exclusion of the bird from the
egg the limits between the altered notochord and the cartilage of protovertebral
origin can only with difficulty be made out.
15. While the chief mass of a proto vertebra, having
given rise to a muscle-plate and a ganglion, is converted into
the body and arch of a permanent vertebra with its several
appurtenances, a small portion of the exterior grows down-
wards as the rudiment for the formation of a rib. These
costal growths are of course confined to the dorsal region.
They are seen on the sixth day as cartilaginous rods, whose
cells are arranged in horizontal rows. By this time they
are quite separate from the bodies of the vertebras, with
whose arches they are in transverse section seen to alternate.
Thus in one section the vertebral arch will be distinctly seen
but no trace of the rib ; while in the next the rib will be
visible but the arch will be absent.
VI.] THE MUSCLE-PLATES. 159
16* We shall conclude our account of the proto vertebra
by describing the changes which take place in the muscle-
plates.
In the chick these are somewhat complicated, and have
not been fully worked out.
On the third day the muscle-plates end opposite the
point where the mesoblast becomes split into somatopleure
and splanchnopleure. OD the fourth day however (Fig.
47 mp.) they extend to a certain distance into the side walls
of the body beyond the point of the division into somatopleure
and splanchnopleure.
Into what muscles of the trunk they become converted has
been somewhat disputed. There is no doubt that it is only epi-
skeletal muscles, to use Professor Huxley's term (Vertebrates,
p. 46), that are derived from them, but some embryologists
have stated that they only form the muscles of the back.
We have, however, little doubt that all the episkeletal
muscles are their products ; a view also adopted by Professors
Huxley and Kolliker.
According to Kolliker the milscle-plates give rise to (i) the deep dorsal
muscles, such as the semispinalis multifidus &c., and (2) the visceral muscles
as represented by abdominal muscles, the muscles of the breast, the superficial
muscles of the neck, and the muscles of the jaws and face.
The front dorso-lateral (hyposkeletal) muscles, according to Kolliker, are
derived from a front (ventral) muscle-plate, which is formed from the most
ventral portion of the protovertebrse, but is very limited in extent in the
fowl. These muscles include the longus colli, the recti antici, and quadratus.
This view differs from that of Huxley, chiefly in considering only the
ventral dorsal muscles as hyposkeletal, and not also the inner visceral muscles.
Huxley believes that all the episkeletal muscles are derived from the muscle-
plates, but does not give an opinion as to the cells of the embryo from
which the hyposkeletal muscles take their origin.
His takes an entirely different view ; he believes that the muscles of the back
only are derived from the muscle-plates, but that the muscles of the sides and
ventral walls of the body are formed from the mesoblast of the somatopleure.
There can be little doubt that the intrinsic muscles of the limbs are not out-
growths from the muscle-plates, but are formed independently in the meso-
blastic tissues of which the limbs are composed.
The origin of the extrinsic limb-muscles is not so certainly known.
The cutaneous muscles are obviously derived from the original mesoblast of
the somatopleure.
It seems very probable (though the subject has not yet been worked out)
that the hyposkeletal voluntary muscles underlying the vertebral column are
derived from the intermediate cell-mass, which originally lies externally to the
protovertebrae, but into which, as we have before said, the cleavage of the
mesoblast does not extend.
In the first instance, as is clear from their mode of origin, the muscle-plates
correspond in number with the protovertebrse, and this condition is permanent
160 THE FOURTH DAY. [CHAP.
in the lower vertebrates, such as fishes, where we find that the lateral muscle is
divided by septa into a series of segments corresponding in number with the
vertebrae.
17. Of all the events of the fourth day, none perhaps are
more important than those by which the rudiments of the
complex urinary and generative systems are added to the
simple Wolffian duct and body, which up to that time are
the sole representatives of both systems.
We saw that the duct arose on the second day as a solid
ridge which subsequently became a tube, lying immediately
underneath the epiblast above the intermediate cell-mass,
close against the upper and outer angles of the proto-
vertebrse, and reaching from about opposite to the fifth
protovertebra away to the hinder end of the embryo.
The exact manner in which it first appears is as yet a
matter of dispute, and in our account of the second day, we
gave the views of the majority of embryologists who have
written on the subject. But it may be considered as quite
certain that the Wolffian duct is formed out of mesoblast-
cells. It is most probable that the ridge is primarily formed
by simple aggregation of cells, and that it is converted into
a tube by its central cells taking on a radiating arrange-
ment round a central hole, which is at first small but rapidly
increases in size. In whatever way it be really formed, we
find before the end of the second day, in the place of the
previous ridge, a duct with a distinct though small lumen.
Waldeyer and some other observers have incorrectly stated
that the lumen is not formed till somewhat later.
At first the duct occupies a position immediately under-
neath the superficial epiblast, but very soon after its forma-
tion the growth of the protovertebrae and the changes which
take place in the intermediate cell-mass, together with the
general folding in of the body, cause it to appear to change
its place and travel downwards (Chap. V. 26). While this
shifting is going on, the cells lining the upper end of the
pleuroperitoneal cavity (the kind of bay which, as seen in
sections, is formed by the divergence of the somatopleure and
splanchnopleure) become columnar, and constitute a distinct
epithelium. This epithelium, which is clearly shewn in Fig.
41, g. e, and is also indicated in Fig. 44, is often called i\\Q ger-
minal epithelium, because some of its cells subsequently take
VI.] THE WOLFFIAN BODY. 161
part in the formation of the ovary. Soon after its appearance,
the intermediate cell-mass increases in size and begins to
grow outwards into the pleuroperitoneal cavity, as a rounded
projection which lies with its upper surface towards the
somatopleure, and its lower surface towards the splanchno-
pleure, but is in either case separated from these layers
by a narrow chink. The Wolffian duct (Fig. 44, Wd, 47,
Wd) travels down, and finally before the end of the third
day is found in the upper part of this projection, near that
face of it which is turned towards the somatopleure.
At, or before, the fourth day, when the duct occupies
this new position, the Wolffian body begins to be formed in
the midst of the intermediate cell-mass.
The structure of the fully developed Wolffian body is
fundamentally similar to that of the permanent kidneys, and
consists essentially of convoluted tubules, commencing in
Malpighian bodies with vascular glomeruli, and opening
into the duct. It is formed as follows.
From the anterior portion of each duct and on its inner
side, diverticula are given out at right angles. These
gradually lengthen, and becoming twisted form the tubules,
while the glomeruli of the Malpighian corpuscles seem to be
derived from cells of the intermediate mass, which also gives
rise to the vascular networks round the tubules.
The tubules, which from their contorted course are in
sections (Figs. 47, 51) seen cut at various angles, possess
an epithelium which is thicker than that of the Wolffian
duct. From this difference it is generally easy to distin-
guish the sections of the tubules from those of the duct.
The glomeruli of the Malpighian bodies are in sections of
hardened embryos usually filled with blood-corpuscles.
In the above statements we have followed Waldeyer (EierstocJc und Hi), but
it ought to be mentioned that the majority of earlier observers have believed
that the tubules arise independently in the mesoblast, and only at a later period
become connected with the duct. The sections which Waldeyer has drawn
seem however strongly to support the view which he has brought forward ;
our own sections also confirm it, and we have noticed that even before the
formation of the tubules, the Wolffian duct exhibits great variations in diameter,
being in some cases crescent-shaped in section, in others round ; this seems
clearly to indicate the giving off of diverticula. Waldeyer's observations have
moreover been since confirmed by other observers.
The Wolffian body, as distinct from the duct, reaches
from about the level of the fifth protovertebra to beyond the
E. 11
162 THE FOURTH DAY. [CHAP.
hind limbs ; but the duct itself is carried on still further
back.
Towards the hind end of the embryo, the projection of
the intermediate cell-mass spoken of above becomes smaller
and smaller, and the Wolffian duct is thus brought nearer to
the splanchnopleure, and in the region of the hind-gut comes
to lie close to the walls of the alimentary canal. On the fourth
day, the two ducts meet and open into two horns, into which
the side- walls of the recently formed cloaca are at that time
produced, one on either side.
As we shall afterwards see, the duct of the permanent
kidneys and Miiller's duct also fall into these two horns of
the cloaca.
The Wolffian bodies thus constituted perform the offices
of kidneys for the greater part of embryonic life. In
the chick they disappear before birth ; but in most of the
Ichthyopsida they remain for life as the permanent kidneys.
18. Near the end of the fourth day, on the outer surface
of the projection formed by the Wolffian body a furrow is
formed immediately below the Wolffian duct by an involution
of the germinal epithelium. This furrow, which is shewn at
M.d in Fig. 47, deepens, and its walls arch over and unite.
In this way a tube is formed, which separates from the
germinal epithelium in the same way that the neural tube
separated from the external epiblast. It is known as the
Duct of Mutter ; of its function we shall speak later on.
This account of the origin of Muller' s duct is due to Waldeyer (loc. tit.},
whose observations have been confirmed by subsequent inquirers. An exami-
nation of our own sections leads us to the same conclusions.
Dr Sernoff (Centralblatt fur Med. Wiss. 27 Jun. 1874) agrees with
Bornhaupt (Untersuchung uber die Entwickelung des Urino-gemtalsy stems beim
Hilhnchen) in considering that the duct of Miiller is formed by a simple invo-
lution from the pleuroperitoneal cavity which grows backwards in the meso-
blast between the Wolffian duct and the germinal epithelium ; and thinks that
Waldeyer is in error in supposing the involution to be in the form of an elon-
gated furrow. This divergence of opinion is not of great importance compared
with the point on which both observers are in agreement, viz. that the duct of
Muller is formed by an involution of the germinal epithelium from the pleuro-
peritoneal cavity.
The formation of the duct of Muller takes place from
before, backwards; but near the hind end of the embryo,
where the germinal epithelium is deficient, the groove to
form the duct becomes an involution which, at first solid
VI.] THE PERMANENT KIDNEYS. 163
but subsequently hollow, bores its way through the meso-
blast, and finally appears to unite on the seventh day
with the Wolffian duct close to the entrance of the latter
into the cloaca. Later on, this state of things becomes
altered; the duct of Miiller opens directly into the cloaca
without first uniting with the Wolffian duct. Its opening
then lies above that of the Wolffian duct, between it and the
opening into the cloaca of the true urinary canal, of which
we shall speak directly.
The anterior extremity of the duct of Miiller which lies
about on a level with the fifth protovertebra is never closed
in. Here the original furrow remains open, and forms a
funnel-shaped opening into the tube from the pleuro-
peritoneal cavity. In sections of the sixth day the duct
of Miiller is to be seen lying between the duct of the
Wolffian body and the pleuroperitoneal cavity. Its diameter
is generally smaller than that of the Wolffian duct.
19. Between the 80th and 100th hour of incubation, the
permanent kidneys begin to make their appearance, and as
is the case with the Wolffian bodies, the first portion of them
to appear is their duct. Near its posterior extremity the
Wolffian duct becomes expanded, and from the expanded
portion a diverticulum is constricted off which in a trans-
verse section lies above the original duct, and the blind end
of which points forwards, that is, towards the head of the
chick. This is the duct of the permanent kidney or ureter.
At first the ureter and the Wolffian duct open by a common
trunk into the cloaca, but this state of things lasts for a
short time only, and by the sixth day the two ducts have
independent openings.
The earlier state of things was overlooked by Remak, who thus came to give
an incorrect account of the origin of the duct of the kidneys.
Knpffer (Untersuchung uber die Entwickelung des Ham- und GeschlecTits-
systems, Archiv fur Microscop. Anat. Vol. n. 1866) was the first to give a
correct account of the development of the duct of the permanent kidneys
in the chick. His observations have since been confirmed by a number of other
observers, including Waldeyer.
In sections of a somewhat later period the duct of the
kidneys can be seen to lie above (dorsal to) the Wolffian
duct, and farther from it than the duct of Miiller.
The formation of the kidneys themselves is very similar
to the formation of the Wolffian bodies,
11 9
164 THE FOUKTH DAY. [CHAP.
From the upper end of the ureter diverticula are given
off at right angles into the intermediate cell-mass. These
lengthening and becoming twisted, form the tubuli urini-
feri, while the mesoblast around their extremities becomes
directly converted into the Malpighian bodies and the
capillary network of the kidneys. Corresponding to the
relative position of their ducts, the kidney lies above the
Wolffian body. At its first appearance it forms an oval
body, lying in the upper part of intermediate cell-mass
between the Wolffian body and the vertebral column, and
is placed rather nearer the median line than the Wolffian
body.
The formation of the kidneys takes place before the end
of the seventh day, but they do not become of functional
importance till considerably later.
From their mode of development it clearly follows that
the permanent kidneys are merely parts of the same system
as the Wolffian bodies, and that their separation from these
is an occurrence of a purely secondary importance.
20. Before describing the subsequent fate of the Wolffian
and Mullerian ducts, it will be necessary to give an account
of the formation of the true sexual glands, the ovaries and
testes.
At the first appearance of the projection from the inter-
mediate mass, which we may now call the genital ridge,
a columnar character is not only visible in the layer of cells
covering the nascent ridge itself along its whole length, but
may also be traced for some little distance outwards on either
side of the ridge in the cells lining the most median portions
of both somatopleure and splanchnopleure. Passing out-
wards along these layers, the columnar cells gradually give
place to a flat tesselated epithelium. As the ridge con-
tinues to increase and project, the columnar character be-
comes more and more restricted to cells covering the ridge
itself, in which at the same time it becomes more distinct.
On the outer side of the ridge, that is on the side which
looks towards the somatopleure, the epithelium undergoes, as
we have seen, an involution to form the duct of Muller, and
for some little time retains in the immediate neighbourhood
of that duct its columnar character (Fig. 51, a'), though
eventually losing it.
THE .GERMINAL EPITHELIUM.
165
The median portion of the ridge is occupied by the
projection of the Wolffian body, and here the epithelium
rapidly becomes flattened.
On the inside of the ridge, however, that is on the side
looking towards the splanchnopleure, the epithelium not only
retains its columnar character, but grows several cells deep
(Fig. 51, a), while at the same time the mesoblast (E) under-
lying it becomes thickened. In this way, owing partly to
the increasing thickness of the epithelium, and partly to
the accumulation of mesoblast beneath it, a slight eminence
is formed, which when viewed from above, after opening the
SECTION OF THE INTERMEDIATE CELL-MASS ON THE FOUBTH DAY. (Prom
Waldeyer.) Magnified 160 times.
m. mesentery. L. somatopleure. a', portion of the germinal epithelium from
which the involution to form the duct of Miiller (2) takes place, a. thickened
portion of the germinal epithelium in which the primitive ova C and o are
lying. E. modified mesoblast which will form the stroma of the ovary.
WK. Wolffian body. y. WolfEan duct.
166 THE FOURTH DAY. [CHAP.
abdominal cavity, appears in direct light as a fusiform white
patch or streak, in its early stages extending along the whole
length of the Wolffian body and genital ridge, but sub-
sequently restricted to its anterior portion. Its appearance
under these circumstances has been well described by Von Baer.
This 'sexual eminence' is present in the early stages of
both sexes. In both the epithelium consists of several layers
of short cylindrical cells, a few of which are conspicuous on
account of their size and their possessing a highly refractive
oval nucleus of considerable bulk ; in both, the underlying
thickened mesoblast consists as indeed at this epoch it does
generally in all parts of the body of spindle-shaped cells.
The larger conspicuous cells of the epithelium, which
appear to have quite a common origin with their fellow cells,
and to rise from them by direct differentiation, and which
are seen at the first in male as well as female embryos, are
the primordial ova (Fig. 51, o). Thus in quite early stages
it is impossible to detect the one sex from the other. At
about the 80th to the 100th hour, however, a distinction
becomes apparent.
In the males, the epithelium with its underlying meso-
blast ceases to develope ; the primordial ova neither increase
nor multiply. On the contrary, they disappear, and the
whole sexual eminence fades away.
In females, on the other hand, the primordial ova enlarge
and become more numerous, the whole epithelium growing
thicker and more prominent. The spindle-shaped cells of
the underlying mesoblast also increase rapidly, and thus
form the stroma of the ovary. The growth of this stroma
bears subsequently such a relation to that of the epithelium,
that the primordial ova appear to sink into the stroma, and
each ovum, as it descends, to carry with it a number of the
ordinary epithelium-cells, which arrange themselves round it
in a distinct layer. In this way each ovum becomes invested
by a capsule of vascular connective tissue, lined internally by
a layer of epithelium ; the whole constituting a Graffian
follicle. The large nucleus of the primordial ovum becomes
the germinal vesicle, while the ovum itself remains as the
true ovum ; this subsequently becomes enlarged by the ad-
dition of a quantity of yolk derived from the epithelial lining
of the follicle.
VI.] THE TESTES. 167
Pfliiger (Die Eierstocke der Sdugethiere u. des Menschen, Leipzig, 1863) de-
scribed the ova as arising, in mammals, out of the epithelium of tubular glands,
a chain of several ova being frequently found in one tube and the tube be-
coming subsequently divided by constrictions into as many follicles. According
to Waldeyer however, whose account we have followed above, the primordial
ova make their appearance as individual specialized epithelium- cells, without the
preformation of any tubular glands, the capsule or Graffian follicle being a later
product. Waldeyer's views have been on the whole generally accepted (Leo-
pold, Untersuch. uber das Epithel. des Ovariums. Inaug. Diss. Leipzig, 1870,
Bomiti, Max Schnitzels Archiv, 1873, Bd. x.), though opposed by Kapff (Rei-
chert and Du ois Reymond's Archiv, 1872), and more recently by Sernoff
(loc. cit.).
The first traces of the testes are found in the dorsal and
inner side of the intermediate cell-mass, and appear about the
sixth day. From the first they differ from the rudimentary
ovaries, by coming into somewhat close connection with
the Wolffian bodies ; but occupy about the same limits from
before backwards. The mesoblast in the position we have
mentioned begins to become somewhat modified, and by the
eighth day is divided by septa of connective tissue into a
number of groups of cells ; which are the commencing tubuli
seminiferi. By the sixteenth day the cells of the tubuli have
become larger and acquired a distinctly epithelial character.
Waldeyer is of opinion that the tubules of the Wolffian body penetrate into
the tissue from which the testes are formed, and becoming much finer than the
remainder of the tubules constitute the Hubuli seminiferi.' Apart from its
inherent difficulties, this view has not been corroborated by any subsequent
observer.
It is distinctly denied by Sernoff (loc. cit.}, who further states that the testes
are entirely formed out of the mesoblast of the intermediate cell-mass, and that
their rudiments have no connection either with the germinal epithelium or with
the tubules of the Wolffian body.
We have now described the origin of all the parts which
form the urinary and sexual systems, both of the embryo and
adult. It merely remains to speak briefly of the changes,
which on the attainment of the adult condition take place in
the parts described.
The Wolffian body, according to Waldeyer, may be said
to consist of a sexual and urinary part, which can, he states,
be easily distinguished in the just-hatched chick. The sexual
part becomes in the cock the after-testes or coni vasculosi,
and consists of tubules which lose themselves on the one
hand in the seminiferous tubules, and on the other hand, in
birds, probably form the whole of what can be called the
epididymis. In the hea it forms part of the parovarium of
168 THE FOURTH DAY. [CHAP.
His, and is composed of well-developed tubes without pig-
ment. The urinary part forms in both sexes a small rudi-
ment, consisting of blindly ending tubes with yellow pigment,
but is most conspicuous in the hen.
The Wolffian duct remains as the vas deferens in the
male. In the female it becomes atrophied and nearly dis-
appears.
The duct of Muller on the right side (that on the left side
with the corresponding ovary generally disappearing) remains
in the female as the oviduct. In the male it is almost
entirely obliterated on both sides.
21. We may return to the changes which are taking
place in the circulation.
On the fourth day, the point at which the dorsal aorta
divides into the two branches which we may now call the iliac
arteries is carried much further back towards the tail.
A short way beyond the point of bifurcation, each iliac
gives off a branch to the newly formed allantois. It is not,
however, till the second half of the fourth day, when the
allantois grows rapidly, that these allantoic, or as we may now
call them umbilical, arteries acquire any importance, if indeed
they are present before. With the increase of the allantois
they speedily acquire such a size, that the iliac trunks from
which they were given off seem to be mere branches of them-
selves.
The omphalo-mesaraic arteries are before the end of the
day given off from the undivided aortic trunk as a single but
quickly bifurcating vessel, the left of the two branches
into which it divides being much larger than the right.
During the third day, we saw that the arterial arch
running in the first visceral fold became obliterated, the
obliteration being accompanied by the appearance of a new
(fourth) arch running in the fourth visceral fold on either
side.
During the fourth day the second pair of arterial arches
also becomes nearly (if not entirely) obliterated; but a new
pair of arches is developed in the last (fifth) visceral fold,
behind the last visceral cleft; so that there are still three
pairs of arterial arches, which however now run in the
third, fourth and fifth visceral folds. The last of these is as
yet small, and together with the slight remains of the second
VI.] THE ARTERIAL ARCHES. 169
pair of arches we may consider that there are in all four
pairs of arches. When the first and second arches are
obliterated, it is only the central portion of each arch on
either side which absolutely disappears. The ventral portion
connected with the bulbus arteriosus, and the dorsal portion
which joins the dorsal aorta, both remain, and are both
carried straight forward towards the head. The ventral
portions of both first and second arches unite on each side to
form a branch, the external carotid (Fig. 52, E. CA), which
runs straight up from the bulbus arteriosus to the head.
FIG. 52.
JE.C+.
T.CA
STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH DAT.
E. CA. external carotid. /. CA. internal carotid. AO. dorsal aorta, wf. A.
arteries to the Wolffian bodies. Ver. A. arteries given off between each of
the vertebrae. Of. A, omphalo-mesaraic artery. UA. umbilical artery.
I A. iliac artery.
In the same way the dorsal portions form a branch, the
internal carotid, which takes its origin from the dorsal or far
end of the third arch.
22. In the venous system important changes also occur.
As the liver in the course of its formation wraps round
the common trunk of the omphalo-mesaraic veins, or meatus
venosus, it may be said to divide that vessel into two parts :
170
THE FOURTH DAY.
[CHAP.
into a part nearer the heart which is called the sinus venosus
(Fig. 53, S.V.\ and into a part surrounded by the liver
which is called the ductus venosus. Beyond, i.e. behind the
liver, the ductus venosus is directly continuous with the
omphalo-mesaraic veins, or as we may now say, vein, for the
right trunk has become so small as to appear a mere branch
of the left. (Fig. 53, Of.)
We saw that on the third day the ductus venosus, while
running through the liver, exhibited numerous
bulgmgs
FIG. 53.
DlAGBAM OF THE VENOUS ClKCULATION AT THE COMMENCEMENT OF THE FlFTH
DAY.
H. heart. 2). O. ductus Cuvieri. Into the ductus Cuvieri of each side fall /. the
jugular vein, or superior cardinal vein, Su. V. the superior vertebral vein, W.
the vein from the wing and C. the inferior cardinal vein. S. V. sinus
venosus. Of. omphalo-mesaraic vein. U. umbilical vein, which at this
stage gives off branches to the body- walls.
VI.] THE VEINS OF THE LIVER. 171
indicative of branches about to be formed. These are on the
fourth day actually formed, and become connected with the
capillary network simultaneously developed in the hepatic
substance in such a way that those branches which come off
from the ductus venosus soon after its entrance between the
liver-lobes, carry blood into the substance of the liver, while
those which join the ductus venosus shortly before it leaves the
liver, carry blood away from the hepatic substance into the
ductus. The former are called vence advehentes, the latter
vence revehentes. As a result of this arrangement, there is a
choice of paths for the blood in passing from the omphalo-
mesaraic vein to the sinus venosus ; it may pass through the
capillary network of the liver, going in by the venae adve-
hentes, and coming back again by the vena3 revehentes, or it
may go straight through the ductus venosus without passing
at all into the substance of the liver.
As the alimentary canal by its continued closing in
becomes on the fourth day more and more distinct from the
yolk-sac, it gradually acquires veins of its own, the mesenterio
veins, which first appear as small branches of the omphalo-
mesaraic vein, though eventually, owing to the change in the
relative size and importance of the yolk-sac and intestine, the
latter seems to be a branch of one of the former.
Corresponding to the increase in the size of the head, the
superior cardinal veins (Fig. 53, */.) become larger and more
important and are joined by the superior vertebral (Su.V.)
and wing veins (W). As before, they form the ductus
Cuvieri (D.C.) by joining with the cardinal veins (C).
The latter are now largely developed ; they seem to take
origin from the Wolffian bodies, and their size and importance
is in direct proportion to the prominence of these bodies.
They might be called the veins of the Wolffian bodies.
As the kidneys begin to be formed, a new single median
vein makes its appearance, running from them forwards,
beneath the vertebral column, to fall into the sinus venosus
(Fig. 53, V.C.I.). This is the vena cava inferior.
As the lungs are being formed, the pulmonary veins also
make their appearance and become connected with the left
side of the auricular division of the heart.
The blood carried to the allantois by the umbilical
arteries is brought back by two veins which very soon after
172 THE FOURTH DAY. [CHAP.
their appearance unite close to the allantois into a single
trunk, the umbilical vein, which, running along the splanch-
nopleure, falls into the omphalo-mesaraic vein (Fig. 53, U).
23. Meanwhile the heart is undergoing considerable
changes. Though the whole organ still exhibits a marked
curvature to the right, the ventricular portion becomes
directed more distinctly downwards, forming a blunted cone
whose apex will eventually become the apex of the adult
heart.
The concave (or dorsal) walls of the ventricles become
much thicker, as did the convex or ventral walls on the third
day.
Well-marked constrictions now separate the ventricles
from the bulbus arteriosus on the one hand, and from
the auricles on the other. The latter constriction is very
distinct, and receives the name of canalis auricularis
(Fig. 54, CL4.) ; the former, sometimes called the /return
Hcdleri, is far less conspicuous.
The most important event is perhaps the formation of
the ventricular septum. This, which commenced on the
third day as a crescentic ridge or fold springing from the
convex or ventral side of the rounded ventricular portion of
the heart, now grows rapidly across the ventricular cavity
towards the concave or dorsal side. It thus forms an in-
complete longitudinal partition extending from the canalis
auricularis to the commencement of the bulbus arteriosus,
and dividing the twisted ventricular tube into two somewhat
FIG. 54.
C.A
HEART OF A CHICK ON THE FOURTH DAY OP INCUBATION VIEWED FROM
THE VENTRAL SURFACE.
1. a. left auricular appendage. C. A. canalis auricularis. V. ventricle, b. bulbus
- - - .... .... ; arteriosus.
VI.] THE VENTRICULAR SEPTUM. 173
curved canals, one more to the left and above, the other to the
right and below. These communicate freely with each other,
above the free edge of the partition, along its whole length.
Externally the ventricular portion as yet shews no division
into two parts.
The bulbus arteriosus (Fig. 54, 6) has increased in size,
and is now very conspicuous.
The venous end of the heart is placed still more dorsal,
and to the left of the arterial end; its walls are beginning to
become thicker.
The auricles are nearly if not quite as far forward as the
ventricles; and the auricular appendages (Fig. 54, l.a.\ which
were visible even on the third day, are exceedingly prominent,
giving a strongly marked external appearance of a division of
the auricular portion of the heart into two chambers; but
Von Baer was unable to detect at this date any internal
auricular septum.
24. The chief events of the fourth day are :
(1) The increase of the cranial and body flexure.
(2) The increase in the tail-fold.
(3) The formation of the limbs as local thickenings of
the Wolffian ridge.
(4) The formation of the olfactory grooves.
(5) The absorption of the partition between the mouth
and the throat.
(6) The formation of the allantois as a diverticulum
of the alimentary canal.
(7) The formation of the spinal ganglia.
(8) The vacuolation of the cells of the notochord.
(9) The formation of the Wolffian body.
(10) The involution of the germinal epithelium to
form the duct of Miiller.
(11) The appearance of the primitive ova in the ger-
minal epithelium.
(12) The development of a fifth pair of arterial arches
and the obliteration of the second pair.
(13) The origin from the ductus venosus of the capil-
laries of the liver.
(14) The development of the 'canalis auricularis,' the
growth of the septum of the ventricles and of the auricular
appendages.
CHAPTER VII.
THE CHANGES WHICH TAKE PLACE ON THE FIFTH DAY.
1. ON opening an egg about the middle of the fifth day,
the observer's attention is not arrested by any new features ;
but he notices that the progress of development, which was so
rapid during the later half of the fourth day, is being con-
tinued with undiminished vigour.
The allantois which on the fourth day began to project
from the pleuroperitoneal cavity has grown very rapidly,
and now stretches away from the somatic stalk far over the
right side of the embryo (which it will be remembered is
lying on its left side) in the cavity between the two amniotic
folds (Fig. 8, K). It is very vascular, and already serves
as the chief organ of respiration.
The blastoderm has spread over the whole of the yolk-
sac ; and the yolk is thus completely enclosed in a bag
whose walls, however, are excessively delicate and easily
torn. The vascular area extends over. about two-thirds of
the yolk.
The splanchnic stalk or umbilical duct has now reached
its greatest narrowness ; it has become a solid cord, and will
undergo no further change till near the time of hatching.
The space between it and the somatic stalk is still con-
siderable, though the latter is narrower than it was on the
fourth day.
2. The embryo remains excessively curved, so much so
indeed that the head and the tail are nearly in contact.
The limbs have increased, especially in length ; in each
a distinction is now apparent between the more cylindrical
CHAP. VII.] THE LIMBS. 175
stalk and the flattened terminal expansion ; and the carti-
laginous precursors of the several bones have already become
visible.
The fore and hind limbs are still exceedingly alike, and
in both the stalk is already beginning to be bent about its
middle to form the elbow and knee respectively.
The angles of both knee and elbow are in the
first instance alike directed outwards and somewhat back-
wards. By the eighth day, however, the elbow has come
to look directly backwards and the knee forwards. In
consequence of this change, the digits of the fore limb
point directly forwards ; those of the hind limb directly
backwards. This state of things is altered by a subsequent
rotation of the hand and foot on the arm and leg, so that by
the tenth day the toes are directed straight forwards, and
the digits of the wing, backwards and somewhat downwards,
the elbow and knee almost touching each other.
While these changes are taking place, the differences
between wing and foot become more and more distinct.
The cartilages of the digits appear on the fifth day as streaks
in the broad flat terminal expansions, from the even curved
edge of which they do not project. On the sixth or seventh
day the three digits of the wing (the median being the
longest), and the four (or in some fowls five) digits of the
foot may be distinguished, and on the eighth or ninth day
these begin to project from the edge of the expanded foot
and wing, the substance of which, thin and more or less
transparent, remains for some time as a kind of web between
them. By the tenth day, the fore and hind extremities, save
for the absence of feathers and nails, are already veritable
wings and feet.
At an early period of development we find the following elements in the
avian manus, as separate masses of cartilage.
In the carpus there are four elements. Two in the proximal row which
remain distinct through life, viz. (i) the radiale, (2) the united intermedium
and ulnare. In the distal row (according to some recent observations of
I)r Rosenberg, Zeitschrift fur Wiss. Zoologie, 1873, p. 139, etc.) there are also
two elements. One of these is the united first and second carpal which we
may call carpal l ~ u , and the other is the united third and fourth carpal which
we may call carpal in ~ lv . These subsequently unite with the metacarpal
bones, and form with them a united ' carpo-metacarpus ' comparable with the
tarso-metatarsus of the avian foot.
Four metacarpals are present, viz. the first, second, third, and fourth. The
176 THE FIFTH DAY. [CHAP.
first, second, and third are the usually recognized elements, and to these
Dr Rosenberg's investigations (loc. cit.) have added a fourth. The first, second
and third persist in the adult, though they become anchylosed in all recent
birds. They also fuse, as we have said above, with the distal, row of the carpals.
Phalanges belonging to the first, second, and third metacarpals are present.
There thus seem in the avian manus to be no representatives of the centrale,
the fifth carpal, the fifth nietacarpal and the phalanges of the fourth and fifth
digits.
Of the elements we have spoken of in the avian hand, the only ones which
require further notice are the carpal 1 " 11 , carpal UI - 1V , and the fourth ineta-
carpal.
The united first and second carpal first appears as a small mass of cartilage
close to the proximal end of the second metacarpal. In this condition it
persists for some time but commences finally to fuse with the first metacarpal ;
and at a slightly subsequent period with the second metacarpal. These rela-
tions with the first and second metacarpals shew without doubt that this
little mass of cartilage is the representative of the first and second bones of
the distal row of the carpus. In a still later stage carpal I ~ 11 fuses also with
carpal in ~ IY . Its distinct nature as a separate element in the bird's manus is
again shewn during ossification, when there appears for it a separate centre of
ossification.
Carpal m " IV appears about the same time as carpal 1 " 11 but is at its first appear-
ance united with metacarpals three and four ; it soon becomes separated from
metacarpal three, and afterwards also from metacarpal four. It subsequently
undergoes considerable changes of shape, and rather later fuses with carpal 1 " 11 .
Its true nature is again, as with carpal 1 " 11 shewn during ossification by the
appearance of a separate centre of ossification for it.
The fourth metacarpal is, as we have described, at first united with carpal m ~ IV ,
but subsequently the neck connecting the two becomes constricted, and finally
they become completely separated from each other. The small independent
mass of cartilage thus formed represents the fourth metacarpal; it applies itself
closely to the side of the third metacarpal, though without becoming united with
it. It ossifies very late some time after the hatching of the chick, and after
ossification fuses with the third metacarpal and then in most cases disappears
completely.
The pes of a fowl in its early embryonic condition consists of
(t) a mass of cartilage close to the distal end of the tibia. It represents
(Gegenbaur) the proximal row of tarsal bones, viz. the l tibiale^ the l inter-
medium^ the 'fibularej and the 'centraleS This cartilage fuses in the adult
with the distal end of the tibia.
(2) a mass of cartilage representing the five bones of the distal row of
the tarsus. In the adult this unites with the metatarsus, forming a tarso-
metatarsus.
(3) the metatarsus. There are usually stated to be four metatarsal bones
present in the metatarsus of a fowl, which are anchylosed in the adult, but are
represented by separate rods of cartilage in the embryo. They are the distal
extremity of a first metatarsal, and complete second, third and fourth metatarsal
bones. In addition to these Dr Rosenberg (loc. cit.) has found a small oval mass
of cartilage representing a fifth metatarsal. Soon after its appearance this
becomes fused with the end of the tarsal mass of cartilage representing the
fifth tarsal, but later entirely atrophies.
(4) There are four phalanges present both in the embryo and the adult, a
number which is never exceeded in birds (except amongst some abnormal breeds
of fowls, e.g. the Dorking fowls) ; though one or more of the four are frequently
deficient.
VII.] THE INVESTING MASS. 177
3. As we mentioned in the last chapter, the formation
of the primitive cranium commenced upon the fourth day.
This in its earliest stage, inasmuch as it is composed of con-
densed but otherwise only slightly differentiated mesoblast,
may be spoken of as the membranous cranium.
On the sixth day, true hyaline cartilage makes its ap-
pearance ; and the primitive membranous cranium gives
place to the primitive cartilaginous cranium.
The cartilage which is the first to appear, forms a thick
plate called the investing mass of Rathke (Fig. 55, iv.) t sur-
rounding the whole of that portion of the notochord which
projects in front of the foremost protovertebra. The hinder
FIG. 55.
VlEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL-E ON
THE FOURTH DAT OF INCUBATION. (From Parker.)
In order to shew this, the whole of the upper portion of the head has been
sliced away. The cartilaginous portions of the skull are marked with the dark
horizontal shading.
cv. i. cerebral vesicles (sliced off), e. eye. nc. notochord. iv. investing mass.
9. foramen for the exit of the ninth nerve, cl. cochlea, hsc. horizontal
semicircular canal, q. quadrate. 5. notch for the passage of the fifth nerve.
Ig. expanded anterior end of the investing mass. pts. pituitary space.
tr. trabeculse. The reference line tr. has been accidentally made to end a
little short of the cartilage.
E. 12
178 THE FIFTH DAY. [CHAP.
portion of this investing mass sends upwards along the sides
of the brain two lateral projections or wings, which enclose
the rudiments of the internal ear. In the chick the portions
which thus inclose the auditory sacs seem never to be at any
time separate from the remainder of the investing mass. At
the front end of the notochord the cartilaginous investing
mass divides into two horizontal branches in the form of two
cartilaginous rods called the trabeculce (Fig. 55, tr.) y which
passing forward (in a somewhat different plane from the
investing mass), meet again in front, and so enclose a space
called the pituitary space pts, into which the infundibulum
extends downward. In front of this junction, the trabeculse
expand into a somewhat broad plate (subsequently developed
into the ethmoid and nasal cartilages), which ends in two
horns in the interior of the fronto-nasal process.
The front end of the notochord probably defines the
anterior boundary of the basi-occipital. At first it extends
quite up to the pituitary space and the starting-point of the
trabeculse. Subsequently, however, there takes place be-
tween it and the pituitary space a growth of cartilage in
which the ossification for the basi- sphenoid takes place.
The lateral projections at the hinder end of the investing
mass grow up behind, and completely enclose that part of
the neural canal from which the medulla oblongata is de-
veloped, and in it ossifications arise to form the occipital
bones and the bones which invest the auditory labyrinth.
It is important to notice that the only segment of the
skull, which primarily forms a cartilaginous roof to any part
of the brain, is the occipital segment. The roof of the re-
mainder of the skull is formed by membrane-bones.
For the histological differences observable in the develop-
ment of cartilage and membrane bones, we must refer
the reader to treatises on histology; for our purpose it is
sufficient to say that a membrane-bone is one which is not
preformed in cartilage, while a cartilage-bone is one in which
the ossification takes place in a bed of cartilage, which fills
the place subsequently occupied by the bone.
The trabeculaa together with the cartilage between the
pituitary space and the end of the notochord give rise to the
sphenoid bone, while in the cartilage in front of the trabeculae
the ethmoid and nasal bones are formed.
VII.] THE VISCERAL ARCHES. 179
From the study of the development of the skull, especially in some of tl>e
lower vertebrates, Mr Parker and Professor Huxley have shewn, that the
trabeculse are developed independently of the investing mass, and that their
subsequent connection with it is due to a secondary process. Professor Huxley
is of opinion that they are to be regarded as the remains of a pair of visceral
arches, corresponding with the other five pah's of arches which we find developed
in the chick. The stage in which they exist as simple visceral arches with a
core of undifferentiated mesoblast is not seen in the chick. They first attract
notice when they become cartilaginous rods.
The ordinary visceral arches are, as we have seen, suffici-
ently .obvious, while as yet their mesoblast is quite undiffer-
entiated ; but in them, as in the trabeculae, rods of cartilage
are subsequently developed and begin to make their appear-
ance about the fifth day.
The first arch, it will be remembered, budded off a
process called the superior maxillary process. The whole
arch, therefore, comes to consist of two parts, viz. a superior
and an inferior maxillary process; in each of these, carti-
laginous rods are developed. In the superior maxillary
process, the rod does not appear till the fifth day. It is
called from its subsequent fate, the ptery go-palatine rod, and
consists of a pterygoid and of a palatine part. In the
inferior maxillary process two developments of cartilage take
place ; one which forms the quadrate in the upper or prox-
imal portion close to the origin of the superior maxillary
process, a second in the lower or distal portion, which goes
by the name of Meckel's cartilage.
Cartilaginous rods are also formed in the second and third
arches. These, which give rise to the hyoids and branchials
respectively, quickly come to lie within the first arch, but
do not form a conspicuous portion of the skeleton of the
face.
4. Closely connected with the development of the skull
is the formation of the parts of the face.
After the appearance of the nasal grooves, on the fourth
day the mouth (Fig. 56 Jlf.) appears as a deep depression
inclosed by five processes. Its lower border is entirely
formed by the two inferior maxillary processes (Fig. 56, F.I),
at its sides lie the two superior maxillary processes S. M,
while above it is bounded by the fronto-nasal process nf.
After a while the outer angles of the fronto-nasal process,
enclosing the termination of the ethmovomerine plate, pro-
ject somewhat outwards on each side, giving the end of the
19 9
180 THE FIFTH DAY. [CHAP.
FIG. 56.
B
A. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAT VIEWED FROM BELOW
AS AN OPAQUE OBJECT. (Chromic acid preparation.)
CH. cerebral hemispheres. FB. vesicle of the third ventricle. Op. eyeball.
nf. naso-frontal process. M. cavity of mouth. S. M. superior maxillary
process of F. r, the first visceral fold (inferior maxillary process). F. 2, F. 3,
second and third visceral folds. N. nasal pit.
In order to gain the view here given the neck was cut across between the
third and fourth visceral folds. In the section e thus made, are Been the
alimentary canal al with its collapsed walla, the neural canal ra.c., the noto-
chord cA., the dorsal aorta AO., and the vertebral veins V.
The incision has been carried just below the upper limit of the pleuroperi-
toneal cavity, consequently a portion of the somatopleure appears at the angle
between the two third visceral folds. Almost embraced by the piece of somato-
pleure is seen the end of the bulbus arteriosus Ao.
In the drawing the nasal groove has been rather exaggerated in its upper
part. On the other hand the lower part of the groove, where it runs between
the superior maxillary process S. M. and the broad naso-frontal process, was in
this particular embryo extremely shallow and indeed hardly visible. Hence
the end of the superior maxillary process seems to join the inner and not, as
described in the text, the outer margin of the nasal groove. A few hours later
the separation of the two would have been very visible.
B. The same seen sideways, to shew the visceral folds. Letters as before.
process a rather bilobed appearance. These projecting portions
of the fronto-nasal process form on each side the inner
margins of the rapidly deepening nasal grooves, and are
sometimes spoken of as the inner nasal processes. The outer
margin of each nasal groove is raised up into a projection
frequently spoken of as the outer nasal process which runs
downwards to join the superior maxillary process, from which,
VII.] THE NASAL LABYRINTH. 181
however, it is separated by a shallow depression. This de-
pression, which runs nearly horizontally outwards towards the
eyeball, is, according to Coste and Kolliker, subsequently con-
verted into the lachrymal duct.
On the fifth day, the inner nasal processes or lower and
outer corners of the fronto-nasal process arching over, unite
on each side with the superior maxillary processes. (Com-
pare Fig. 57, which, however, is a view of the head of a
chick , of the sixth day.) In this way each nasal groove is
converted into a canal, which leads from the nasal pit above,
into the cavity of the mouth below, and places the two in
direct communication. This canal, whose lining consists of
epiblast, is the rudiment of the nasal labyrinth.
FIG. 57.
HEAD OF A CHICK AT THE SIXTH DAY FROM BELOW. (Copied from Huxley's
Elements of Comparative Anatomy.)
la. cerebral vesicles, a. eye, in which the remains of the choroid slit can still be
seen. g. nasal pits. k. fronto-nasal process. I. superior maxillary process.
I. inferior maxillary process or first visceral arch. 2. second visceral arch.
x. first visceral cleft between the first and second visceral arches.
The cavity of the mouth is seen enclosed by the fronto-nasal process, the
superior maxillary processes and the first pair of visceral arches. At the back
of it is seen the opening leading into the throat. The nasal grooves leading
from the nasal pits to the mouth are already closed over and converted into
canals. .
By the seventh day (Fig. 58), not only is the union of
the superior maxillary and fronto-nasal processes completed,
and the upper boundary of the mouth thus definitely con-
182 THE FIFTH DAY. [CHAP.
stituted, but these parts begin to grow rapidly forward, thus
deepening the mouth, and giving rise to the appearance of a
nose or beak (Fig. 58), which, though yet blunt, is still
distinct. The whole of the lower boundary of the buccal
cavity is formed by the inferior maxillary processes.
FIG. 58.
HEAD OF A CHICK OF THE SEVENTH DAT FROM BELOW. (Copied from Huxley's
Elements of Comparative Anatomy.)
I a. cerebral vesicles, a. eye. g. nasal pits. Tc. fronto-nasal process. I. superior
maxillary process. I. first visceral arch. 2. second visceral arch. x. first
visceral cleft.
The external opening of the mouth has become much constricted, but it is
fitill enclosed by the fronto-nasal process and superior maxillary processes above,
and by the inferior maxillary process (first pair of visceral arches) below.
The superior maxillary processes have united with the fronto-nasal process,
along the whole length of the latter, with the exceptio" of a small space in
iront, where a narrow angular opening is left between the two.
As we have before mentioned, the ethmovomerine car-
tilage is developed in the fronto-nasal process, the pterygo-
palatine bar in the superior maxillary process, Meckel's
cartilage and the quadrate in the inferior maxillary process ;
the other bones which form the boundaries of the mouth in
the adult are developed later after all external trace of these
parts as separate processes has disappeared. For an account
of their formation, however, we must refer the reader to the
chapter upon the development of the skull.
At first the mouth is a simple cavity into which the
nasal canals open directly. When however the various
VII.] THE MOUTH. 18S
processes unite together to form the upper boundary of the
mouth, each superior maxillary process sends inwards a lateral
bud. These buds become flattened and form horizontal plates
which stretch more and more inward towards the middle line.
There they finally meet, and by their union, which is effected
first in front, and thence extends backwards, they con-
stitute a horizontal plate, stretching right across the mouth,
and dividing it into two cavities an upper and a lower one.
In the front of the mouth their union is quite complete,
so that here there is no communication between the two
cavities. Behind, however, the partition is not a complete
one ; so that the two divisions of the buccal cavity com-
municate at the back of the mouth. The external opening
of the mouth passes into the lower of these two cavities,
which may therefore be called the mouth proper. Into the
upper chamber the nasal ducts open; it maybe called the
respiratory chamber and forms the commencement of the
chamber of the nose. In birds generally the upper nasal
cavity becomes subsequently divided by a median partition
into two chambers, which communicate with the back of the
mouth by separate apertures. The original openings of the
nasal pits remain as the nostrils.
5. One important occurrence of the fifth day is the
appearance of the anus, which is formed very much in the
same way as the mouth.
Beneath the tail an involution of the epiblast takes place
towards the cloaca. At this point the wall of the cloaca,
which has here taken no share in the cleavage of the meso-
blast, becomes thinner, and is finally perforated. An orifice
thus places the cloaca in communication with the exterior,
and constitutes the anus.
6. On this day also important changes take place in
the spinal cord ; and a brief history of the development of
this organ may fitly be introduced here.
At the beginning of the third day, the cavity of the
neural canal (Fig. 41) is still of considerable width, and when
examined in vertical section its sides may be seen to be nearly
parallel, though perhaps approximating to each other more
below than above.
We say below and above, because a vertical section is naturally examined
with its dorsal side uppermost. In the ordinary terminology of the spinal cord,
184 THE FIFTH DAY. [CHAP.
above would be posterior and below anterior. These latter terms it will be
henceforward most convenient to adopt.
The exact shape/however, varies according to the region
of the body from which the section is taken.
The epiblast walls are at this time composed of radiately
arranged columnar cells. The cells are much elongated, but
s-omewhat irregular; and it is very difficult in sections to
make out their individual boundaries. They contain granu-
lar oval nuclei in which a nucleolus can almost always be
seen. The walls of the canal are both anteriorly and pos-
teriorly considerably thinner than in the middle.
Towards the end of the third day, changes take place in
the shape of the cavity. In the lumbar region its vertical
section becomes more elongated, and at the same time very
narrow in the middle while expanded at each end into a some-
what bulbous enlargement, producing an hour-glass appear-
ance (Fig. 44). Its walls however still preserve the same
histological characters as before.
On the fourth day (Fig. 47) coincidently with the appear-
ance of the spinal nerves, important changes may be observed
in the hitherto undifferentiated epiblastic walls.
In the anterior region of the cord, the external portions of
the epiblast become modified into grey matter, forming an
anterior grey column, which in turn is covered superficially by
a mass of white matter forming an anterior white column.
The internal portions of the epiblast remain as the epithe-
lium lining the spinal canal. Both columns are formed at
the point of entrance of the anterior nerve-roots ; and these
may easily be traced through the white into the grey matter.
The grey column is composed of numerous small nuclei,
each of which appears to be surrounded by a definite mass of
protoplasm, though the boundaries of the protoplasm belong-
ing to each nucleus can only occasionally be made out.
The nuclei lie in the meshes of a network of fibres continu-
ous with the fibres of the nerve-root, and passing through
the mass of grey matter in two directions : (1) round the
anterior end of the spinal canal, immediately outside its
epithelium and so to the grey matter on the opposite side,
forming in this way an anterior commissure through which a
decussation of the fibres from the opposite sides is effected :
(2) upwards along the outside of the lateral walls of the canal.
VII.] THE SPINAL CORD. 185
The posterior roots of the spinal nerves enter the cord
near its posterior surface, and at this point the posterior grey
columns are formed in a similar way to the anterior. In
some cases .also the rudiment of a posterior white column
may be seen at the junction of the nerve with the epiblast of
the canal. The fibres of the posterior root cannot be traced
so far into the cord as those of the anterior root.
The grey matter of the cord seems undoubtedly to be formed by a meta-
morphosis of the external cells of the epiblast of the neural tube, and is
directly continuous with the epithelium ; there being no strong line of demarcation
between them. Whether the fibres which traverse it, and which seem to l>e
partly nervous and partly connective tissue in their nature, are derived from
inesoblast or epiblast our observations have not enabled us to determine.
The white matter which caps the grey mass, and which forms the com-
mencement of the anterior white column, is a peculiar tissue. It consists of a
network of fibres somewhat resembling the connective tissue network of the
white matter of the adult cord, to which it has a further likeness in not being
easily stained by carmine. The fibres of which it is composed have a general
tendency to be disposed in radiating septa, a peculiarity which is especially
noticeable with low powers. Along the fibres and more especially in the septa,
numerous highly refracting granules are embedded, and in the meshes pale
spherical nuclei with nucleoli are to be seen. The boundary between the white
and grey matter is very sharply defined, and we have always failed to trace the
fibres of which we are speaking into the fibres present in the grey matter, though
Lockhart Clarke (Phil. Trans. 1862) asserts that they are continuous. Nor can
the fibres of the nerve-roots be seen to come into connection with these same
fibres. It has generally been assumed that the white matter like the grey is
derived from the epiblast : this does not however appear ever to have been
clearly proved, while the peculiarities of the tissue, and the fact that it first
appears at the origin of the spinal nerves, might seem to indicate that it is
directly derived from the mesoblast surrounding the cord ; a view which we are
inclined to accept. .
On the fourth day there is no trace of either an anterior
or a posterior fissure, and in the lumbar region the shape of
the spinal canal is not very different from what it was on the
third day. It appears in sections as a narrow slit dilated
somewhat at either end (Fig. 47). The epithelium surround-
ing the slit is still very thin, especially above and below, but
at the anterior end forms a somewhat arched projection
with the convex surface turned downwards.
On the fifth and sixth days important changes take place.
By the great increase of the grey matter, which now
comes to form the chief mass of the cord, the epithelium is
reduced to a thin layer of cells immediately surrounding the
canal.
In the dorsal region, the side walls of the laterally com-
186 THE FIFTH DAY. [CHAP.
pressed canal come into absolute contact in the middle. So
that sections no longer shew an hour-glass cavity, but two
more or less elliptical cavities, representing the former term-
inal enlargements, one anterior and one posterior, separated
by a neck in which the epithelium of the one side is closely
applied to that of the other. In other words, the original
single canal has been divided longitudinally into an anterior
and posterior canal. Of these the anterior will alone remain
as the permanent central canal of the spinal cord. In the
lumbar region this division has as yet not taken place.
The anterior white columns have very much increased
in quantity ; the posterior white columns have also become
distinct, and the two form together a thick covering for the
grey matter. The two columns of each side are continuous
with each other, but their line of junction is clearly marked ;
and on the sixth day there may be seen at this spot a small
mass of white matter, differing somewhat from the rest in
appearance, which perhaps may be looked upon as the first
commencement of the lateral column. The columns of the
one side are not continuous with those of the other either
posteriorly or anteriorly. In other words, there are as yet
no white commissures.
The anterior ends of the cord on each side of the middle
line have commenced to grow downwards. These outgrowths,
in which both the white and the grey matter take part,
have an important function. They enclose between them a
somewhat linear space : the commencement of the anterior
fissure. This, which is at first not very deep and rather wide,
may be noticed already on the fifth day (L. Clarke) and on
the sixth day is very clearly marked.
Corresponding with these grosser changes, certain histo-
logical features make their appearance. Between the an-
terior and posterior parts into which the grey matter is
divided on each side, or, as we may now call them, the
anterior and posterior cornua, there is found a rather lighter
band of grey matter in which the nuclei are somewhat more
scattered. The anterior cornu exhibits a further division
into an outer and upper part, and a lower and inner part,
in both of which the nuclei are more numerous than in the
intervening mass. The posterior cornu is of considerably
darker colour than the anterior, the difference being due to
VII.] THE POSTERIOR FISSURE. 187
the greater number of nuclei present in the former. The
outlines of the cells are more clearly marked and somewhat
more angular in shape than they were on the fourth day.
The distinctions between the several parts of the grey matter are chiefly
brought about by variations in the number of nuclei in a given area. Throughout
the cord fibres of the grey matter seem to be continuous with the epithelium of
the neural canal, but this is much more strongly marked in the posterior than in
the anterior region. In the posterior region also, it is still much more difficult
to trace the roots of the nerves than in the anterior.
Of the three columns into which the white matter on
each side is divided, the anterior column differs from the
posterior in being thicker and also in having wider meshes
and fewer granules. The lateral column is the most granular
of all and very conspicuous. The minute structure of the
white matter remains about the same as on the fourth day.
Meanwhile an alteration is taking place in the external
outline of the cord. From being, as on the fourth and fifth
days, oval in section, it becomes, chiefly through the increase
of the white matter, much more nearly circular.
On the seventh day the most important event is the
formation of the posterior fissure.
This is brought about by the absorption of the roof of
the posterior of the two parts into which the neural canal
has become divided.
Between the posterior horns of the cord, the epithelium
forming the roof of the, so to speak, posterior canal is
along the middle line covered neither by grey nor by
white matter, and on the seventh day is partially absorbed,
thus transforming the canal into a wedge-shaped fissure,
whose mouth however is seen in section to be partially
closed by a triangular clump of elongated cells (Fig. 59 c.).
Below this mass of cells the fissure is open. It is separated
from the 'true spinal canal ' by a very narrow space along
which the side walls have coalesced. In the lumbar and
sacral regions the two still communicate.
We thus find, as was first pointed out by Lockhart Clarke,
that the anterior and posterior fissures of the spinal cord
are, morphologically speaking, entirely different. The ante-
rior fissure is merely the space left between two lateral
downward growths of the cord, while the posterior fissure is
part of the original neural canal separated from the rest of
188
THE FIFTH DAY.
FIG. 59.
[CHAP,
SECTION THROUGH THE SPINAL CORD OP A SEVEN DAYS' CHICK.
p.C. w. posterior white column. Z. c. w. lateral white column, a. c. w. anterior
white column, p c. posterior corn u, of grey matter, consisting of very
small cells, a. c. anterior cornu of grey matter, with a peculiar mass of
very large cells, ep. epithelium lining the original medullary canal, p.f. pos-
terior fissure. The posterior fissure is chiefly formed by the upper portion
of the original medullary canal which becomes open above. The upper
portion of it is now filled with tissue (c) which is probably derived from
the epithelium of the medullary canal. The lower portion of the medullary
canal becomes the spinal canal (sp. c.) and is eventually entirely shut off
from posterior fissure. The communication between the spinal canal and the
posterior fissure is already narrowed, and if the section had been made
further forwards, the two would have been entirely separated from each
other.
a. /. anterior fissure. This is formed in an entirely different manner from
the posterior fissure. It is produced by the anterior column of white,
and the anterior cornu of grey matter, growing downwards and leaving
between them a fissure. It is at this time filled up with connective
tissue.
a. g. c. anterior grey commissure, c. tissue filling up the end of the posterior
fissure, sp. c. spinal canal. Only the right half of the cord is represented
in the figure. The section passes through the cord between the entrance
of two spinal nerves. The angular form of the cells of the cord has not
been done justice to by the engraver.
VII.] THE WHITE COLUMNS. 189
the cavity (which goes to form the true spinal canal) by
a median coalescence of the side walls.
The lateral white columns have on the seventh day
increased in size and become less granular, and the lines of
junction between them and the anterior columns have now to
be arbitrarily selected. The posterior white columns are
still much thinner and more granular than the anterior. The
nuclei of the white matter are more numerous than before.
Some of the septa of the white matter can now be traced in the one
direction into the grey matter, and in the other direction into the connective
tissue around the cord. Whether these are nerve-fibres which have separated
irom the remainder of the fibres, to enter the cord at a different point, or are
merely trabecuiae of connective tissue, cannot be absolutely determined. The
latter view however seems most probable. In the grey matter, the anterior
and posterior divisions are better distinguished than at an earlier date. In
particular the nuclei of the cells of the posterior division are both smaller and
more numerous than those of the anterior. Some of the fibres from the
posterior root, after entering the grey matter, quickly pass out again into the
posterior column of the white matter.
In the anterior division of the grey matter, near the entrance of the anterior
roots, there is a peculiar and well-marked mass of somewhat triangular cells,
with large and distinct nuclei, more deeply stained with carmine than the
remainder of the grey matter. This mass of cells is present in the lumbar and
sacral regions, but is deficient or very inconspicuous in the dorsal portion of the
cord. The nuclei of the whole anterior region of the grey matter have increased
in size, and the cells to which they belong (when clearly visible) are usually
iound to be angular.
Around the true spinal canal, the line of separation between the epithelium
and the grey matter is sharply defined, but elsewhere is very indistinct.
By the end of the seventh day, the following important
parts of the cord have been definitely established :
(1) The anterior and posterior fissures.
(2) The anterior and posterior horns of grey matter.
(3) The anterior, posterior and lateral columns of white
matter.
(4) The spinal canal.
As yet, however, the grey mass of the two sides of the
cord only communicate by the anterior grey commissure, and
the white columns of opposite sides do not communicate
at all. The grey matter, moreover, still far preponderates
over the white matter in quantity.
By the ninth day the posterior fissure is fully formed,
and the posterior grey commissure has also appeared.
In the centre of the sacral enlargement this commissure
is absent, and the posterior columns at a later period separate
190 THE FIFTH DAY. [CHAP.
widely and form the ' sinus rhomboidalis,' which is not, as
has been sometimes stated, the remains of the primitive
'sinus rhomboidalis' visible during the second day.
The anterior white columns have much increased on this
day, and now form the sides of the already deep anterior
fissure. The anterior white commissure does not however
appear till somewhat later.
7. The fifth day may perhaps be taken as marking a
most important epoch in the history of the heart. The
changes which take place on that and on the sixth day,
added to those previously undergone, transform the simple
tube of the early days of incubation into an almost com-
pletely formed heart.
The venous end of the heart, though still lying somewhat
to the left and above, is now placed as far forwards as the
arterial end, the whole organ appearing to be drawn together.
The ventricular septum is complete.
The apex of the ventricles becomes more and more
pointed. In the auricular portion a small longitudinal fold
appears as the rudiment of the auricular septum, while in
the canalis auricularis, which is now at its greatest length,
there is also to be seen a commencing transverse partition
tending to separate the cavity of the auricles from those of
the ventricles.
About the 106th hour, a septum begins to make its
appearance in the bulbus arteriosus in the forrn of a longitu-
dinal fold, which according to Dr Tonge (Proc. of Royal Soc.
1868) starts, not (as Von Baer thought) at the end of the
bulbus nearest to, but at that furthest removed from, the
heart. It takes origin from the wall of the bulbus between
the fifth and fourth pairs of arches and grows downwards in
such a manner as to divide the bulbus into two channels, one
of which leads from the heart to the fourth and third pair of
arches and the other to the fifth pair. The free edge of the
septum is somewhat V-shaped, so that its two legs as it were
project downwards towards the heart, further than its central
portion ; and this shape of the free edge is maintained
during the whole period of its growth. Its course downwards
is not straight but spiral, and thus the two channels into
which it divides the bulbus arteriosus, wind spirally the one
over the other. The existence of the septum can only be
VII.] THE SEMILUNAR VALVES. 191
ascertained at this stage by dissection or by sections, there
being as yet no external signs of the division.
At the time when the septum is first formed, the opening
of the bulbus arteriosus into the ventricles is narrow or slit-
like, apparently in order to prevent the flow of the blood
back into the heart. Soon after the appearance of the septum,
however, semilunar valves (Tonge, loc. cit.) are developed
from the wall of that portion of the bulbus which lies between
the free edge of the septum and the cavity of the ventricles.
These arise as six solid outgrowths of the wall arranged in
pairs, an anterior, an inner, and an outer pair, one valve of
each pair belonging to the one and the other to the other of
the two main divisions of the bulbus which are now being
established.
The anterior and the inner pairs of valves are the first to
appear : the former as two small solid prominences separated
from each other by a narrow groove ; the latter as a single
shallow ridge, in the centre of which is a prominence indi-
cating the point where the ridge will subsequently become
divided into two. The outer pair of valves appear opposite
each other, at a considerably later period, between the ends
of the other pair of valves on each side.
As the septum grows downwards towards the heart, it
finally reaches the position of these valves. One of its legs
then passes between the two anterior pair of valves, and the
other unites with the prominence on the inner valve-ridge.
At the same time the growth of all the parts causes the
valves to appear to approach the heart and thus to be placed
quite at the top of the ventricular cavities. The free edge of
the septum of the bulbus now fuses with the ventricular
septum, and thus the division of the bulbus into two separate
channels, each provided with three valves, and each com-
municating with a separate side of the heart, is complete,
the position of the valves not being very different from what
it is in the adult heart.
That division of the bulbus which opens into the fifth pair
of arches is the one which communicates with the right
ventricle, while that which opens into the third and fourth
pairs communicates with the left ventricle. The former
becomes the pulmonary artery, the latter the commencement
of the systemic aorta.
192
THE FIFTH DAY.
[CHAP.
The external constriction actually dividing the bulbus
into two vessels, does not begin to appear till the septum has
extended some way back towards the heart.
The semilunar valves become pocketed at a period con-
siderably la'ter than their first formation (from the 147th to
the 165th hour) in the order of their appearance.
8. Towards the end of the fifth and in the course of the
sixth day further important changes take place in the heart.
The venous end with its two very conspicuous auricular
appendages, comes to be situated more above (dorsal to) the
arterial end, though it still turns rather towards the left.
The venous portion of the heart undergoes on the sixth day
or even near to the end of the fifth, such a development of the
muscular fibres of its walls, that the canalis auricularis becomes
almost entirely concealed. The point of the heart is now
TWO VIEWS OF THE HEART OF A CHICK UPON THE FlFTH DAY OF INCUBATION,
A from the ventral, B from the dorsal side.
/. a. left auricular appendage, r. a. right auricular appendage, r. x. right
ventricle, l.v. left ventricle. 6. bulbus arteriosus.
VII.] THE BULBUS ARTERIOSUS. 193
directed nearly backwards (i.e. towards the tail), but also a
little downwards.
An alteration takes place during the sixth day in the
relative position of the parts of the ventricular division of
the heart. The right ventricle is now turned towards the
abdominal surface, and also winds to a certain extent round
the left ventricle. It will be remembered that on the fourth
day the right ventricle was placed above (dorsal to) the left.
The right ventricle is now also the smaller of the two,
and the constriction which divides it from the left ventricle
does not extend to the apex of the heart (Fig. 60) It has,
however, a very marked bulge towards the right.
At first the bulbus arteriosus appeared to come off
chiefly from the left ventricle ; during the fifth day, and
still more on the sixth, it appears to come from the right
chamber. This is caused by the canal from the right ven-
tricle into the bulbus arteriosus passing towards the left,
and on the ventral side, so as entirely to conceal the origin of
the canal from the left chamber of the heart. On the seventh
day the bulbus arteriosus appears to come less markedly from
the right side of the heart.
All these changes, however, of position of the bulbus
arteriosus only affect it externally; during the whole time
the two chambers of the heart open respectively into the two
divisions of the bulbus arteriosus. The swelling of the
FIG. 61.
T.O,
T.V
HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION, FBOM THE
VENTRAL SURFACE.
I. a. left auricular appendage, r.a. right auricular appendage, r.v. right ventricle,
I. v. left ventricle, b. bulbus arteriosus.
E. 13
194 THE FIFTH DAY. [CHAP.
bulbus is much lestf marked on the seventh day than it was
before.
At the end of the sixth day, and even on the fifth day
(Figs. 60, 61), the appearance of the heart itself, without
reference to the vessels which come from it, is not very
dissimilar from that which it presents when adult.
The original curvature to the right now forms the apex
of the ventricles, and the two auricular appendages are
placed at the anterior extremity of the heart.
The most noticeable difference (in the ventral view) is
the still externally undivided condition of the bulbus arte-
riosus.
About the sixth or, perhaps, even on the fifth day, the
pericardium, according to Von Baer, makes its appearance.
Its mode of formation is not exactly known, but it probably
takes origin from folds of the lining of the thoracic cavity
which meet and coalesce.
9. The subsequent changes which the heart undergoes
are concerned more with its internal structure than with its
external shape. Indeed, during the next three days, viz. the
eighth, ninth, and tenth, the external form of the heart re-
mains nearly unaltered.
In the auricular portion however, the septum which com-
menced on the fifth day becomes now more conspicuous. It
is placed vertically, and arises from the ventral wall ; com-
mencing at the canalis auricularis and proceeding backwards,
it does not as yet reach the opening into the sinus venosus.
The blood from the sinus, or, as we may call it, the
inferior vena cava, enters the heart obliquely from the right,
so that it has a tendency to flow towards the left auricle of the
heart, which is at this time the larger of the two.
The valves between the ventricles and auricles are now
well developed, and it is about this time that the division of
the bulbus arteriosus into the aorta and pulmonary artery
becomes visible on the exterior.
By the eleventh or thirteenth day the right auricle has
become as large as the left, and the auricular septum much
more complete, though there is still a small opening, the
foramen ovale, by which the two cavities communicate
with each other. Through this foramen the greater part of
the blood of the vena cava inferior, which is now joined just
VII.] THE EUSTACHIAN VALVE. 195
at its entrance into the heart by the right vena cava superior,
is directed into the left auricle. The left vena cava superior
enters the right auricle independently ; between it and the
inferior vena cava is a small valve which directs its blood
entirely into the right auricle.
On the sixteenth day the right vena cava superior, when
viewed from the exterior, still appears to join the 'inferior
vena cava before entering the heart ; from the interior how-
ever the two can now be seen to be separated by a valve.
This valve, called the 'Eustachian valve/ extends to the
opening of the left vena cava superior, and into it the valve
which in the earlier stage separated the left superior and
inferior venae cava3 has apparently become merged. There is
also on. the left side of the opening of the inferior cava a
membrane, stretching over the foramen ovale, and serving as
a valve for that orifice. The blood from the inferior cava
still passes chiefly into the left auricle through the foramen
ovale ; while the blood from the other two venae cava3 now
falls into the right auricle, being prevented from entering
the left chamber by the Eustachian valve.
Hence, since at this period also the blood from the left
ventricle passes to a great extent to the anterior portion
of the body, there is a species of double-circulation going
on. The greater part of the blood from the allantois entering
the left auricle from the inferior vena cava passes into the
left ventricle and is thence sent chiefly to the head and
anterior extremities ; from these it is brought back through
the right auricle to the right ventricle, from whence it is
returned along the aorta to the allantois.
From the seventeenth to the nineteenth day, the right
auricle becomes larger than the left. The large Eustachian
valve still prevents the blood from the superior cavae from
entering the left auricle, while it conducts the blood from the
inferior vena cava into that chamber through the foramen
ovale. The entrance of the inferior vena cava is however
further removed than it was from the foramen ovale, and the
increased flow of blood from the lungs prevents all the blood
)f the inferior cava from entering into the left auricle. At
the same time the valve of the foramen ovale prevents the
blood in the left auricle from entering the right auricle.
During the period from the seventh day onwards, the
13-2
196 THE FIFTH DAY. [CHAP.
apex of the heart becomes more marked; the arterial roots
are more entirely separated and the various septa completed,
so that when the foramen ovale is closed and the blood of
the inferior vena cava thereby entirely confined to the
right auricle, the heart has practically acquired its adult con-
dition.
10. The fifth day may also be taken as marking the
epoch at which histological differentiation first becomes
distinctly established.
It is of course true that long before this date, even from
the earliest hours, the cells in each of the three fundamental
layers have ceased to be everywhere alike. Nevertheless the
changes undergone by the several cells have been few and slight.
The cells of epiblastic origin, both those going to form the
epidermis and those included in the neural involution, are up
to this time simple more or less columnar cells ; they may be
seen here elongated, there oval, and in another spot spheroidal ;
here closely packed with scanty protoplasm, there scattered
with each nucleus well surrounded by cell-substance ; but
wherever they are found they may still be recognized as cells
of a distinctly epithelial character. So also with the cells
of hypoblastic origin, whether simply lining the alimentary
canal or taking part in the formation of the compound
glands. Even in the mesoblast, which undergoes far more
changes than either of the other layers, not only increasing
more rapidly in bulk but also serving as the mother tissue
for a far greater number of organs, the alterations in the
individual cells are, till near upon the fifth day, insignificant.
Up to this time, the mesoblast may be spoken of as consisting
of little more than indifferent tissue : of nuclei imbedded in
a protoplasmic cell-substance. In one spot the nuclei are
closely packed together, and the cell-substance scanty and
compact ; at another the nuclei are scattered about with
spindle-shaped masses of protoplasm attached to each, and
there is a large development either of intercellular spaces or
of intracellular vacuoles, filled with clear fluid. The proto-
plasm differs in various places, chiefly in being more or less
granular, and less or more transparent, having as yet under-
gone but slight chemical transformation. Up to this epoch
(with the exception of the early differentiated blood), there
are no distinct tissues, and the rudiments of the various
VII.] THE EPIBLAST. 197
organs are simply marked out by greater or less condensation
of the simple mesoblastic substance.
From the fifth day onwards, however, histological differ-
entiation takes place rapidly ; and it soon becomes possible
to speak of this or that part as being composed of muscular,
or cartilaginous, or connective &c. tissue. It is not within
the scope of the present work to treat in detail of these histo-
genetic changes, for information concerning which we would
refer the reader to histological treatises. We have already
had occasion to refer incidentally to many of the earliest
histological events ; and shall content ourselves by giving a
brief summary of the derivation of the tissues of the adult
animal from the three primary layers of the blastoderm.
The epiblast or upper layer of many embryologists forms
primarily two very important parts of the body, viz. the
central nervous system and the epidermis.
It is from the involuted epiblast of the neural tube, that
the whole of the grey matter of the brain and spinal cord
appears to be developed, the simple columnar cells of the
epiblast being apparently directly transformed into the
characteristic caudate nerve-cells. There is, however, some
doubt whether mesoblast cells may not possibly enter into
its formation, and it is very probable that the white matter
of the brain and spinal cord is derived from the mesoblast
alone.
The epithelium (ciliated in the young animal) lining the
canalis centralis of the spinal cord, together with that lining
the ventricles of the brain, all which cavities and canals are,
as we have seen, derivatives of the primary neural canal, is
the undifferentiated remnant of the primitive epiblast.
The epiblast as we have said also forms the epidermis ;
not however the dermis, which is of mesoblastic origin.
The line of junction between the epiblast and the meso-
blast coincides with that between the epidermis and the
dermis. From the epiblast are formed all such tegumentary
organs or parts of organs as are epidermic in nature.
In addition to these, the epiblast plays an important
part in the formation of the organs of special sense.
According to their mode of formation, these organs
may be arranged into two divisions. In the first come the
eases where the sensory expansion of the organ of special
198 THE FIFTH DAY. [CHAP.
sense is derived from the involuted epiblast of the medullary
canal. To this class belongs the Retina, including the epi-
thelial pigment of the choroid, which is formed from the original
optic vesicle budded out from the fore-brain.
To the second class belong the epithelial expansions of
the membranous labyrinth of the ear, and the cavity of the
nose, which are formed by involution from the superficial
epiblast covering the external surface of the embryo. These
accordingly have no primary connection with the brain. We
may also fairly suppose that the 'taste bulbs' and the nervous
cells which have lately been described as present in the
epidermis are also structures formed from the epiblast.
In addition to these we have the crystalline lens formed of
involuted epiblast and the cavity of the mouth lined by it.
These are the most important parts which are derived from
the epiblast.
From the hypoblast are derived the epithelium of the
digestive canal, the epithelium of the trachea, bronchial
tubes and air cells, the cylindrical epithelium of the ducts
of the liver, pancreas and other glands of the alimentary canal,
as well as the hepatic cells constituting the parenchyma of
the liver, developed as we have seen from the solid hypoblast
cylinders given off around the primary hepatic diverticula.
Homologous probably with the hepatic cells, and equally
of hypoblastic origin, are the spheroidal i secreting cells' of
pancreas and other glands. The epithelium of the salivary
glands, though these so exactly resemble the pancreas,
is of epiblastic origin, inasmuch as the cavity of the mouth
(Chap. VI. 8) is entirely lined by epiblast.
The hypoblast also lines the allantois.
From the mesoblast are formed all the remaining parts
of the body. The muscles, the bones, the connective tissue
and the vessels, both arteries, veins, capillaries and lymphatics
with their appropriate epithelium, are entirely formed from
the mesoblast.
All the nerves of the body, both the cranial nerves (the
so-called optic and olfactory nerves alone excepted), the
spinal nerves, "and the sympathetic system, are also formed
from the mesoblast. The nerve-cells of the sympathetic
ganglia as well as those of the ganglia on the posterior roots
of the spinal nerves are of mesoblastic origin, and thus appar-
VII.] THE MESOBLAST. 199
ently are in striking contrast with the nerve-cells in the brain
and cord. The fibres constituting the white matter of both
brain and spinal cord are also probably derived from mesoblast.
The generative and urinary organs are entirely derived
from the mesoblast. It is worthy of notice that their epithe-
lium, though resembling so closely the hypoblastic epithelium
of the alimentary canal, is distinctly mesoblastic.
From the mesoblast lastly are derived all the muscular,
conr^ective, and nervous and vascular elements, as well of the
alimentary canal and its appendages as of the skin and the
tegumentary organs. Just as it is only the epidermic moiety of
the latter which is derived from the epiblast, so it is only the
epithelium of the former which comes from the hypoblast.
In the present state of our knowledge we cannot in all
cases with certainty say which parts of the mesoblast enter
into the formation of particular organs ; the more important
facts in this part of our subject will however already have
been gathered, from the earlier part of this work.
11. The important events then which characterize the
fifth day are:
1. The growth of the allantois.
2. The appearance of the knee and elbow, and of the
cartilages which precede the bones of the digits and limbs.
3. The formation of the primitive cartilaginous cranium,
more especially of the investing mass, the trabeculse, and the
ethmo-vomerine plate ; and the appearance of rods of cartilage
in the visceral arches.
4. The developments of the parts cf the face: the
closing in of the nasal passages by the nasal processes.
5. The formation of the anus.
6. A large development of grey matter in the spinal
cord as the anterior and posterior cornua ; considerable growth
both of the anterior and posterior white columns, and the
commencement of the anterior and posterior fissures.
7. The appearance of the auricular septum, of a septum
in the bulbus arteriosus, and of the semilunar valves.
8. The establishment of the several tissues.
CHAPTER VIII.
FROM THE SIXTH DAY TO THE END OF INCUBATION.
1. THE sixth day marks a new epoch in the development
of the chick, for distinctly avian characters then first make
their appearance. '
Striking and numerous as are the features, which render
the class aves one of the most easily recognizable in the whole
animal kingdom, the embryo of a bird does not materially
differ in its early phases from that of a reptile or a mammal,
even in the points of structure which are most distinctively
avian. It may, it is true, be possible to infer, even at a com-
paratively early stage, from some subsidiary tokens, whether
any given embryo belongs to this class or that (and indeed the
same inference may be drawn from the ovum itself) ; but up to
a certain date it is impossible to point out, in the embryo of
the fowl, the presence of features which may be taken as
broadly characteristic of an avian organization. This absence
of any distinctive avian differentiation lasts in the chick
roughly speaking till the commencement of the sixth day.
We do not mean that on the sixth day all the organs
suddenly commence to exhibit peculiarities which mark
them as avian. There are no strongly marked breaks in the
history of development; its course is. perfectly gradual, and
one stage passes continuously into the next. The sixth and
seventh days do however mark the commencement of the
period in which the specialization of the bird begins to be
apparent. Then for the first time there become visible
the main features of the characteristic manus and pes ; the
crop and the intestinal caeca make their appearance; the
stomach takes on the form of a gizzard; the nose begins
CHAP. VIII.] THE FCETAL APPENDAGES. 201
to develope into a beak ; and the commencing bones of the
skull arrange themselves after an avian type. Into these
details we do not propose to enter, and shall therefore
treat the history of the remaining days with great brevity.
We will first speak of the FCETAL APPENDAGES.
2. On the sixth and seventh days, these exhibit changes
which are hardly less important than the events of previous
days.
The amnion at its complete closure on the fourth day
very closely invested the body of the chick ; the true cavity of
the amnion was then therefore very small. On the fifth day,
fluid begins to collect in the cavity, and raises the membrane
of the amnion to some distance from the embryo. The
cavity becomes still larger by the sixth day, and on the
seventh day is of very considerable dimensions, the fluid
increasing with it. On the sixth day Von Baer observed
movements of the embryo, chiefly of the limbs ; he attributes
them to the stimulation of the cold air on opening the
egg. By the seventh day very obvious movements begin
to appear in the amnion itself; slow vermicular contractions
creep rythmically over it. The amnion in fact begins to
pulsate slowly and rythmically, and by its pulsation the
embryo is rocked to and fro in the egg. This pulsation is
due probably to the contraction of involuntary muscular
fibres, which seem to be present in the attenuated portion
of the mesoblast, forming part of the amniotic fold. (Cf.
Chap. n. 9, p. 42.) Similar movements are also seen in
the allantois at a considerably later period.
The growth of the allantois has been very rapid, and it
forms a flattened bag, covering the right side of the embryo
and rapidly spreading out in all directions, between the
primitive folds of the amnion, that is between the amnion
proper and the false amnion (or chorion). It is filled with
fluid, so that in spite of its flattened form its opposite walls
are distinctly separated from each other.
The vascular area has become still further extended than
on the previous day, but with a corresponding loss in the
definite character of its blood-vessels. The sinus terminalis
has indeed by the end of the seventh day lost all its
previous distinctness. And the vessels which brought back
the blood from it to the heart are no longer to be seen,
202 THE SIXTH DAY. [CHAP.
Both the omphalo-mesaraie arteries and veins now pass
to and from the body of the chick as single trunks, assuming
more and more the appearance of being merely branches of
the mesenteric vessels.
The yolk is still more fluid than on the previous day,
and its bulk has (according to Von Baer) increased. This
can only be due to its absorbing the white of the egg, which
indeed is diminishing rapidly.
3. During the eighth, ninth, and tenth days, the
amnion does not undergo . any very important changes.
Its cavity is still filled with fluid, and on the eighth day
its pulsations are at their height, henceforward diminishing
in intensity.
The splitting of the mesoblast has now extended to the
outer limit of the vascular area, viz. over about three quarters
of the yolk-sac. The somatopleure at this point is continuous
(as can be easily seen by reference to Fig. 8) with the
original outer fold of the amnion.
It thus comes about that the further splitting of the
mesoblast merely enlarges the cavity in which the allantois
lies. The growth of this organ keeps pace with that of the
cavity in which it is placed. Spread out over the greater
part of the yolk-sac as a flattened bag filled with fluid, it
now serves as the chief organ of respiration.
Hence it is very vascular, the vessels on that side of the
bag which is turned to the chorion and shell being especially
large and numerous.
The yolk now begins to diminish rapidly in bulk. The
yolk-sac becomes flaccid, and on the eleventh day is thrown
into a series of internal folds, abundantly supplied with
blood-vessels. By this means the surface of absorption is
largely increased, and the yolk is more and more rapidly
taken up by the blood-vessels, and in a partially assimilated
condition transferred to the body of the embryo.
4. By the eleventh day the abdominal parietes though
still much looser and less firm than the walls of the chest
may be said to be definitely established, and the loops of
intestine, which have hitherto been hanging down into the,
somatic stalk, are henceforward confined within the cavity
of the abdomen. The body of the embryo is therefore
completed ; but it still remains connected with its various
VTII.] THE ALLANTOIS. 203
appendages by a narrow somatic umbilicus, in which run
the stalk of the allantois and the solid cord suspending
the yolk-sac.
The cleavage of the rnesoblast still progressing, the yolk
is completely invested by the (splanchnopleuric) yolk-sac
except at the pole opposite to the embryo, where for some
little time a small portion remains unenclosed ; at this spot
the diminished white of the egg adheres as a dense viscid
plug,
The allantois meanwhile spreads out rapidly, and lies
over the embryo close under the shell, being separated from
the shell membrane by nothing more than an attenuated
membrane the chorion, formed out of the outer primitive
fold of the amnion and the remains of the vitelline mem-
brane. With this chorion the allantois partially coalesces,
and in opening an egg at the later stages of incubation,
unless care be taken the allantois is in danger of being torn
in the removal of the shell membrane. As the allantois in-
creases in size and importance, the umbilical (or allantoic)
vessels are correspondingly developed. They are very con-
spicuous when the egg is opened, the pulsations of the
umbilical arteries at once attracting attention.
5. On about the sixteenth day, the white having en-
tirely disappeared, the cleavage of the mesoblast is carried
right over the pole of the yolk opposite the embryo, and
is thus completed (Fig. 8). The yolk-sac now, like the
allantois which closely wraps it all round, lies loose in a
space bounded outside the body by the chorion, and con-
tinuous with the pleuroperitoneal cavity of the body of the
embryo. Deposits of urates now become abundant in the
allantoic fluid.
The loose and flaccid walls of the abdomen enclose a
space which the empty intestines are far from filling, and on
the nineteenth day the yolk-sac, diminished greatly in bulk
but still of some considerable size, is withdrawn through the
somatic stalk into the abdominal cavity, which it largely
distends. Outside the embryo there remains nothing now
but the highly vascular allantois and the practically blood-
less chorion and amnion. The amnion, whose fluid during
the later days of incubation rapidly diminishes, is continuous
at the umbilicus with the body-walls of the embryo. The
204 THE SIXTH BAY. [CHAP.
chorion, (or outer primitive amniotic fold,) is by the comple-
tion of the cleavage of the mesoblast and the invagination
of the yolk-sac, entirely separated from the embryo. The
cavity of the allantois by means of its stalk passing through
the umbilicus is of course continuous with the cloaca.
6. In the EMBRYO itself a few general points deserve
notice.
By the sixth or seventh day, the flexure of the body has
become less marked, so that the head does not lie so near
to the tail as on the previous days; at the same time a
more distinct neck makes its appearance.
Though the head is still disproportionately large, its
growth ceases to be greater than that of the body.
Up to this period the walls of the somatic stalk have
remained thin and flaccid, almost membranous in fact, the
heart appearing to hang loosely out of the body of the
embryo. About this time however the stalk, especially in
front, rapidly narrows and its mesoblast becomes thickened.
In this way the heart and the other thoracic viscera are en-
closed by definite firm chest walls, along the sides of which
the ribs grow forwards and in front of which the cartilaginous
rudiments of the sternum appear.
The abdominal walls are also being formed, but not to
the same extent, and the stalk of the allantois still passes
out from the peritoneal cavity between the somatic and the
splanchnic stalks.
In the brain one of the most marked features is the
growth of the cerebral hemispheres. The median division
between these has in front increased in depth, so that
the lateral ventricles are continued forwards as two divergent
horns, while backwards they are also continued as similar
divergent horns separated from one another by the vesicle of
the third ventricle.
We propose to treat more fully of the development of the brain in a later
part of this work, the importance of the mammalian brain rendering it un-
desirable to go too much into the details of the brain of the bird.
All the visceral clefts, with the exception of the first, are
closed by the seventh day: this one however still remains
open, communicating with the mouth by the Eustachian tube
and with the exterior by the aperture of the external auditory
VIII.] THE FEATHERS. 205
meatus. It becomes divided internally into two parts by the
tympanic membrane.
The structures which surround the mouth are beginning
to become avian in form, though the features are as yet not
very distinctly marked. The inferior maxillary processes
meet in front and form the lower boundary of the mouth ;
while, separated from these by only a narrow slit, the superior
maxillary processes and fronto-nasal process meet in a similar
way above, to form the upper boundary. The union of the
superior maxillary processes is not with the tip of the fronto-
nasal process, but with its sides, so that an angular space is
left on each side between them (vide Fig. 58). The nasal
grooves are however completely roofed over.
The tongue has appeared on the floor of the mouth as a
bud of mesoblast covered by epiblast.
7. During the eighth, ninth, and tenth days, the
embryo grows very rapidly, the head being still especially
large, and at the same time becoming more round, the mid-
brain not being so prominent.
8. From the eleventh day onwards the embryo suc-
cessively puts on characters which are not only avian,- but
even distinctive of the genus, species and variety.
So early as the ninth or tenth day the sacs containing
the feathers begin to protrude from the surface of the skin as
papillae especially prominent at first along the middle line of
the back from the neck to the rump, and over the thighs,
the sacs of the tail feathers being very conspicuous. On the
thirteenth day, these sacs, generally distributed over the
body, and acquiring the length of a quarter of an inch or
more, appear to the naked eye as feathers, the thin walls of
the sacs allowing their contents, now coloured according to
the variety of the bird, to shine through. They are still
however closed sacs, and indeed remain such even on the
nineteenth day when many of them are an inch in length.
On the eighth day a chalky-looking patch is observable
on the tip of the nose. This by the twelfth day has become
developed into a horny but still soft beak.
On the thirteenth day, nails are visible at the ex-
tremities, and scales on the remaining portions of the toes.
These on the sixteenth day become harder and more horny,
as does also the beak.
206
THE SIXTH DAY.
[CHAP.
By the thirteenth day the cartilaginous skeleton is com-
pleted and the various muscles of the body can be made out
with tolerable clearness.
Ossification begins according to Von Baer on the eighth
or ninth day by small deposits in the tibia, in the metacarpal
bones of the hind- limb, and in the scapula. On the eleventh or
twelfth day a multitude of points of ossification make their
appearance in the limbs, in the scapular and pelvic arches, in
the ribs, in the bodies of the cervical and dorsal vertebraB and
in the bones of the head, the centres of ossification of the
vertebral arches not being found till the thirteenth day.
FIG. 62.
- DIAGRAM OF THE VENOUS CIRCULATION AT THE COMMENCEMENT OF THE
FIFTH DAY.
JI. heart. D. C. ductus Cuvieri. Into the ductus Cuvieri of each side fall /. the
jugular vein, Su. V. the superior vertebral, W. the vein from the wing and
C. the inferior cardinal vain. S. V. sinus venosus. Of. omphalo-mesaraic
vein. U. umbilical vein, which at this stage gives off branches to the
body- walls. V. 0. 1. inferior vena cava.
VIII.] THE VENA CAVA INFERIOR. 207
9. The events which we have thus briefly narrated are
accompanied by important changes in the arterial and
venous systems.
The condition of the venous system at about the end
of the third day was fully described in Chap. V. 16, and
the changes which have taken place between that date and
the latter days of incubation may be seen by comparing
the diagram Fig. 39 B with the diagrams Figs. 62 and 63.
On the third day, nearly the whole of the venous blood
from the body of the embryo was carried back to the heart by
two main venous trunks, the superior (Fig. 39 B, Su. V.) and
inferior (Fig. 39 B, (7) cardinal veins, joining on each side
to form the short transverse ductus Cuvieri, both which in
turn united with the sinus venosus close to the heart. As
the head and neck continue to enlarge and the wings become
developed, the single superior cardinal or jugular vein,
as it is usually called (Fig. 62, J"), of each side, is joined
by two new veins : the vertebral vein (Su. V.), bringing
back blood from the head and neck, and the vein from the
wing (W).
The inferior cardinal or vertebral veins have their roots
in the Wolffian bodies ; they become developed, pari passu,
with those organs, and may be called the veins of the
Wolffian bodies. On the third day they are the only veins
which bring the blood back from the hinder part of the body
of the embryo.
About the fourth or fifth day, however, a new single
venous trunk, the vena cava inferior (Fig. 62, V. C. /.), makes
its appearance in the middle line, in a plane more dorsal
than that of the cardinal veins. This, starting from the
sinus venosus not far from the heart, is on the fifth day a
short trunk running backward in the middle line below the
aorta, and speedily losing itself in the tissues above the
Wolffian bodies. When the kidneys are formed it receives
blood from them, and thenceforward enlarging rapidly be-
comes the channel by which the greater part of the blood
from the hind limbs and the hinder part of the body finds its
way to the heart. In proportion as this vena cava inferior
increases in size, and the Wolffian bodies give place to the
permanent kidneys, the posterior cardinal veins diminish.
Communicating branches between them and it are established
208
THE SIXTH DAY.
FIG. 63.
[CHAP.
-JSVY.
DIAGRAM OF THE VENOUS CIRCULATION DURING THE LATER DAYS OP
INCUBATION.
II. heart. V. S. R. right vena cava superior. V. S. L. left vena cava superior.
S. V. sinus venosus. The two venae cavee superiores are the original
'ductus Cuvieri/ they still open into the sinus venosus and not in-
dependently into the heart. /. jugular vein. 817. V. superior vertebral
vein. W. vein for the wing. V. C.I. vena cava inferior, which receives
most of the blood from the inferior extremities, etc. HP. Hepatic veins,
which fall into the vena cava inferior. D. V. ductus venosus. P. V. portal
vein. M. a vein bringing blood from the intestines into the portal vein.
Of. omphalo-mesaraic vein. U. umbilical vein. The three last mentioned
veins unite together to form the portal vein.
The remnants of the inferior cardinal veins are not shewn.
VIII.] THE VEN.E CAV^E. 209
in the substance of the Wolffian bodies, so that soon the
blood of those organs, as well as that from all the rest of the
hind body, with the exception of the alimentary canal and its
appendages, passes into the vena cava inferior.
The diminished trunks of the cardinal veins remain for
some time ; their anterior ends unite to form the small azygos
vein.
At its first appearance the vena cava inferior may be con-
sidered as a branch of the trunk which we have called the
sinus venosus, but as development proceeds, and the vena
cava becomes larger and larger, the sinus venosus assumes
more and more the appearance of being merely the cardiac
termination of the vena cava, and the ductus venosus may
now be said to join the vena cava instead of being prolonged
into the sinus.
While this growth of the vena cava is going on, the points
at which the ductus Cuvieri enter into the sinus venosus, or,
as we may now say, vena cava inferior, are drawn in towards
the heart itself, and finally these trunks fall directly and
separately into the auricular cavities, and are henceforward
known as the right and left vena cava superior (Fig. 63, V.S.R.,
V.S.L.}. There are therefore, when these changes have been
effected, three separate channels, with their respective orifices,
by which the blood of the body is brought back to the heart,
viz. the right and left superior and the inferior venae cavae.
While the auricular septum is as yet unformed, the blood
from these veins falls into both auricles, perhaps more into
the left than into the right. As the septum however grows
up, the three vessels become connected with the right auricle
only while the left receives the two pulmonary veins coming
from the lungs. (Compare Chap. vn. 7.)
On the third day the course of the vessels from the yolk-
sac is very simple. The two omphalo-mesaraic veins, of which
the right is already the smaller, form the meatus venosus
from which, as it passes through the liver on its way to the
heart, are given off the two sets of vence advehentes and venae
revehentes.
With the appearance of the allantois on the fourth day,
a new feature is introduced. From the .meatus venosus,
a short distance behind the liver, there is given off a vein
which quickly divides into two branches. These, running
E. 14
210 THE SIXTH DAY. [CHAP.
along the ventral side of the body from the walls of
which they receive some amount of blood, pass to the
allantois. They are the allantoic or umbilical veins. The
single vein which they unite to form becomes, by reason
of the rapid growth of the allantois, very long ; and hence
it is perhaps better to speak of it as the umbilical vein
(Fig. 63, u). The right branch soon diminishes in size
and finally disappears. Meanwhile the left on reaching the
allantois bifurcates ; and, its two branches becoming large
and conspicuous, there still appear to be two main allantoic
or umbilical veins uniting at a short distance from the
allantois to form the single long umbilical vein. At its
first appearance the umbilical vein- seems to be but a small
branch of the omphalo-mesaraic, but as the allantois grows
rapidly, and the yolk-sac dwindles, this state of things is
reversed, and the less conspicuous omphalo-mesaraic appears
as a branch of the larger umbilical.
On the third day the blood returning from the walls
of the intestine is insignificant in amount. As however the
intestine becomes more and more developed, it acquires a
distinct venous system, and the blood sent to it by branches
of the aorta is returned by veins which form a trunk, the
mesenteric vein (Fig. 63, M), falling into the omphalo-mesaraic
vein at its junction with the umbilical vein.
These three great veins in fact, viz. the omphalo-mesaraic,
the umbilical, and the mesenteric, form a large common trunk
which enters at once into the liver, and which we may now
call the portal vein (Fig. 63, P. V.}. This, at its entrance into
the liver, partly breaks up into the vence advehentes, and partly
continues as the ductus venosus straight through the liver,
emerging from which it joins the vena cava inferior. Before
the establishment of the vena cava inferior, the venae reve-
hentes, carrying back the blood which circulates through the
hepatic capillaries, joined the ductus venosus close to its exit
from the liver. By the time however that the vena cava has
become a large and important vessel it is found that the
venae revehentes or as we may now call them the hepatic veins
have shifted their embouchment and now fall directly into
that vein, the ductus venosus making a separate junction
rather higher up (Fig. 63, HP).
This state of things continues with but slight changes till
VIII.]
THE VENOUS CIKCULATION.
FIG. 64.
211
DIAGRAM OF THE VENOUS CIRCULATION OP THE CHICK AFTER THE COMMENCE-
MENT OF RESPIRATION BY MEANS OF THE LUNGS.
W. wing- vein, /.jugular vein. Su. V. superior vertebral vein. These unite
together on each side to form the corresponding superior vena cava.
L. V. pulmonary veins. V. C. I. vena cava inferior. HP. hepatic veins.
P. V. portal vein. M. mesenteric veins. At U. and O.f. are shewn the
points at which the umbilical and omphalo-mesaraic veins originated
previous to their becoming obliterated at the commencement of respiration.
Co. V. connecting vessel between the branches of the portal vein and the
vena cava inferior. It is called the coccygeo-mesenteric vein, and unites
the cross branch connecting the two hypogastrics with the mesenteric
vein. It is represented in the figure in a purely diagrammatic manner.
The ductus venosus has become obliterated. The three venae cavse fall
independently into the right auricle and the pulmonary veins into the
left auricle.
142
212
THE SIXTH BAY.
[CHAP.
near the end of incubation, when the chick begins to breathe
the air in the air-chamber of the shell, and respiration is no
longer carried on by the allantois. Blood then ceases to flow
along the umbilical vessels ; they become obliterated. The
omphalo-mesaraic vein, which as the yolk becomes gradually
absorbed proportionately diminishes in size and importance,
comes to appear as a mere branch of the portal vein. The
ductus venosus becomes closed, remaining often as a mere
ligament ; and hence the whole of the blood coming through
the portal vein flows into the substance of the liver, and so
by the two hepatic veins into the vena cava (Fig. 64, HP).
Previous to these changes one of the veins passing from
the rectum into the vena cava has given off a branch which
effects a junction with one of the mesenteric veins. This now
forms a somewhat conspicuous connecting branch between the
systems of the vena cava and the portal vein (Fig. 64, Co. V.).
All three venae eava3 now fall exclusively into the right
auricle, and by the closure of the foramen ovale the blood
J.CA.
I.CA
VA-
STATB OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH BAT,
E. CA. external carotid. /. CA. internal carotid. AO. dorsal aorta, icf. A*
arteries to the Wolffian bodies. Ver. A. arteries given off between each
pair of vertebrae. Of. A. omphalo-mesaraic artery. UA. umbilical artery.
I A . iliac artery.
VIIL] THE CAKOTIDS. 213
flowing through them is entirely shut off from the left auricle,
into which passes the blood from the two pulmonary veins
(Fig. 64,i,F.).
Such is the history of the veins in the chick. As will be
seen in the second part of this work, the course of events in
the mammal, though in the main similar, differs in some
unimportant respects.
10. It remains for us to speak of the changes which have
in the meantime been taking place in the arterial system. The
condition of things which exists on the fifth or sixth day is
shewn in the diagram (Fig. 65).
We have already seen (Chap. VI. p. 168) that of the three
aortic arches which make their appearance on the third day,
the first two disappear : the first on the fourth, the second on
the fifth day ; but that their disappearance is accompanied
by the formation behind them of two new aortic arches, the
fourth and the fifth. Thus there are generally three, never
more than three, pairs of aortic arches present and functional
at one time.
According however to Yon Baer this is not strictly true. He states that
there are four arches present both on the fourth and fifth days. In the case
of the fourth day, a slight remnant of the first pair of arches still persists when
the fourth pair is already formed ; and on the fifth day the third pair has not
entirely disappeared when the fifth pair is formed. In both of these cases
however the first pair of arches of the four is only present for a very short time,
and then is so diminished in size as to be of no importance.
The first pair of arches, before it entirely disappears, sends
off on each side two branches towards the head. Of these,
one forms the direct continuation of the bulbus arteriosus in
a straight line from the point where the first aortic arch
leaves it. Primarily distributed to the tongue and inferior
maxillary region, it becomes the external carotid (Fig. 65,
The other, starting from the point where the aortic arch
of each side joins its fellow above the alimentary canal to form
the dorsal aorta, is primarily distributed to the brain, and
becomes the internal carotid (Fig. 65, 1.CA.).
When the first arch disappears, the external carotid
arteries still remain as the anterior continuations of the
bulbus arteriosus. And since the dorsal trunks uniting the
distal ends of the first and second arches do not become
obliterated at the time when the first pair of arches dis-
214 THE SIXTH DAY. [CHAP.
appears, the internal carotids remain as branches springing
from the distal ends of the second pair of arches ; they are
supplied with blood from that pair, the stream in which flows
chiefly towards the head instead of backwards towards the
dorsal aorta, as is the case with the succeeding arches. When
the second pair of arches is obliterated, the connecting branch
with the next arch is again left, and thus the internal carotids
appear as branches' from the distal ends of the third pair of
arches.
On the third day, the dorsal aorta does not long remain
single in its backward course along the body, but soon
divides into two trunks which run one on either side of the
middle line of the body. On the fourth day however, the
point at which the aorta divides is carried very much further
back, .quite to the posterior end of the Wolffian bodies. The
two branches into which it there divides, form the origin
of the iliac (Fig. 65, 1 A) arteries supplying the hind limbs.
Each of these sends a short branch to the allantois (UA}.
As the allantois grows rapidly and becomes an import-
ant respiratory organ, these allantoic or umbilical arteries
increase so much in size that they speedily appear to be the
direct continuations of the aorta, and the iliac arteries to be
mere branches of them. As a general, though apparently
not invariable rule, the right umbilical artery gets gradually
smaller and soon disappears.
From the main trunk of the aorta are given off small
transverse branches between the vertebrae (represented dia-
grammaticallyin Fig. 65, Ver. A.}, and also important branches
to the Wolffian bodies (Fig. 65, wf. A.}.
The omphalo-mesaraic artery ( Of. A.) now leaves the aorta
as a single but quickly bifurcating trunk, which at the end
of the fifth day is still very large.
By the fifth day, the ventricular portion of the heart
(compare Chap. vn. 7) is almost completely divided into
two chambers. The bulbus arteriosus is also divided by a
septum into two channels, which do not however run in s a
straight course, but have, according to Von Baer, a spiral
arrangement. One of the channels communicates with the
right ventricle of the heart and the other with the left. The
spiral of the former turns from the right and above (dorsal),
to the left and below (ventral) ; the latter from the left and
VIII.] THE SEPTUM OF THE BULBUS ARTEKIOSUS. 215
below (ventral), to the right and above (dorsal). The septum
separating these two channels, according to Dr Tonge (Proc.
Roy. Soc. 1868), commences as an outgrowth from the wall
between the fourth and fifth pair of arches, and is so arranged
that the channel from the left ventricle communicates with
the third and fourth pairs of arches only, and that from the
right ventricle with the fifth pair only.
According to Dr Tonge's view the two channels after the completion of the
septum do not communicate with each other at any point, and are completely
shut off from each other at their ends. Von Baer believed that the two
channels (into which the bulbus arteriosus is divided by the septum) opened at
their distal terminations into a common trunk, but that the direction of the
openings of the two channels caused the two streams of blood from them to
enter into different arterial arches. According to him the stream from the
channel communicating with the left ventricle is directed so as entirely to
miss the last pair of arterial arches, and to fall into the third and fourth pairs,
while that from the right ventricle enters the fifth pair alone.
One result of this arrangement is that all the blood which
passes to the anterior extremity of the body, comes from the
left ventricle of the heart.
At about the seventh day an entire separation begins to
take place between the arterial roots which come respectively
from the right and left chambers of the heart. The root
from the right chamber (Fig. 66, R.P.A.) remains connected
with the fifth pair of arches. The root from the left ventricle
is connected with the third and fourth pairs of arches.
According to Von Baer the right arterial root is connected with the fourth
arch on the left side, and the fifth arch on the right side. He also believed
that the fifth arch on the left side has by this day altogether disappeared.
According to his view, therefore, the pulmonary arteries are derived from
both the fifth and fourth pairs of arches. Rathke (Denkschriften der AJcademie
zu Wien, 1857, Bd. xin.) however altogether denies this, and states that both
pulmonary arteries are derived from the fifth pair of arches alone. We have
in our account followed Rathke's statements.
The lower part of the body still receives blood from both
the right and left ventricles, since the blood which enters
the fifth arch still flows into the common dorsal aorta.
As the lungs however increase in size, a communication
is set up between them and the fifth pair of arches in the
shape of two vessels which, springing one from the arch of
each side, grow downwards towards the lungs. At first
small and narrow, these pulmonary arteries, for such they
are, grow rapidly larger and larger, so that more and more
of the blood from the right ventricle is carried to the lungs.
216
THE SIXTH DAY.
[CHAP.
FIG. 66.
JS.CA
DIAGRAM OF THE CONDITION OF THE ARCHES OF THE AORTA TOWARDS THE
CLOSE OF INCUBATION.
J* 2 ) 3>4> 5- The several aortic arches. E.CA. External carotid. I.Ca. Internal
carotid. O. C.A. Common carotid. V.A. 'Vertebral' artery. J2.sc. Eight
subclavian. L.sc. Left subclavian. L.I.N. Left innominate. AO. Aorta.
P. A. Pulmonary arteries. R.P.A. Right arterial root, or division of
bulbus arteriosus, or pulmonary artery ; the left root or division, con-
stituting the aorta, is seen by its side. RC.b. Communication on the
right side between the fourth and fifth arches. L.C.b. Communicating
trunk between the fifth arch and the dorsal aorta.
At the same time the connection between the third and
fourth pairs of arches on each side grows weaker ; and finally
the passage between them becomes obliterated, so that less
and less of the blood which flows along the third pair of
arches is able to pass backwards to the hind end of the
body.
On the eighth day (according to Rathke, loc. cit.) from the common root of the
external and internal carotids of each side, a branch (Fig. 66, V.A.) is given
off" which passes forwards along the neck, and ends in front by becoming
connected with a branch from the external carotid. This vessel is of
much larger calibre at its two extremities, than in the central part of its
course. In the adult it terminates anteriorly by anastomosing with the
occipital artery (a branch from the external carotid). The smaller calibre of
the central part is still more marked in the adult, than at the stage we are
VIII.] THE SUBCLAVIAN ARTERIES. 217
describing. This vessel "passes up in the neural arch of the vertebrae, and is
usually spoken of as the vertebral artery.
Rathke calls the vertebral arteries the 'arterise collaterals colli,' and if his
view of their development is correct they can scarcely be considered homologous
with the vertebral arteries usually so called in mammals. Soon after the ana-
stomosis between the third and fourth arches disappears, the common trunk of
the carotids on each side becomes much lengthened, and it is from near the base
of the lengthened common carotid that the 'vertebral artery' (as we shall call
Rathke's ' arteria collateralis colli ') takes its origin.
The fourth arch of the right side now becomes the most
important of all the arches ; and nearly the whole of the
blood supplying the hinder parts of the body passes through
it. It is this arch which remains as the permanent aortic
arch of the adult; and it is important to notice that the
arch which forms the great dorsal aorta in birds is the fourth
on the right side, and not as in mammals the fourth on the
left side.
From the observations of Rathke we know, with tolerable certainty, the
manner in which the carotids and the so-called vertebral arteries of birds are
developed. There is however still some doubt as to the origin of the subclavian
arteries, although Von Baer and Rathke have both investigated the point.
Von Baer believed that the artery which forms the continuation of the third
arch, which on Rathke's authority we have called the * internal carotid,'
became the 'vertebral artery;' and he believed that the subclavian was given
off as a branch from it. As Rathke points out, this does not agree with the
anatomy of the parts, since in birds the subclavian forms the continuation of
the innominate artery, after a common branch for the vertebral and carotids
has been given off from it. If it had not been for Rathke's satisfactory
observations on the development of the carotids and vertebrals, this would not
be a fatal objection to Von Baer's view; since we might easily suppose that
although the subclavian was originally a branch of the vertebral, yet by sub-
sequent changes, the point at which the subclavian left the vertebral was
carried further and further back, till finally the subclavian became a branch
of the common trunk of the vertebral and carotids, or in other words the
subclavian formed the continuation of the innominate artery, after the com-
mon branch which divides into the vertebral and carotids had been given off
from it.
Rathke's view of the origin of the subclavian is founded on the analogy of
other vertebrates, rather than on his own observations on the chick. He states
that although he attempted to do so, he was unable satisfactorily to observe the
origin of these vessels in the chick. The following is the view which he adopts,
and which we have followed in our diagram (Fig. 66).
The right subclavian (R.sc.) arises, he believes, either from the connecting
branch (anastomosis) between the fourth and fifth arches (of that side), or from
the branch connecting the fifth arch with the dorsal aorta ; probably from the
former. This would make its development very nearly similar to the develop-
ment of the corresponding subclavian (i.e. the left) in mammals. We shall
mention directly, in speaking of the final changes which the arterial system
undergoes, how it is that the right subclavian finally comes to form the continu-
ation of the right innominate artery.
Tha left subclavian (L. sc.) forms the continuation of the fourth arch
218 THE SIXTH DAY. [CHAP.
of the left side, and primarily takes its origin from the connecting branch
between the fourth and fifth arches. Its mode of development thus nearly
agrees with that of the right subclavian of mammals. In support of this view
of the development of the left subclavian, is the fact that in some birds there
is, between it and the dorsal aorta, a fibrous connection which is the remains of
the vessel which originally carried the blood from this fourth arch to the dorsal
aorta. On the left side therefore, as can easily be understood by reference to
the diagram, the subclaviau without further alteration remains as the continu-
ation of the left innominate, L.I.N.
In consequence of these changes the condition of the
aortic arches during the latter days of incubation, before
respiration by the lungs has commenced, is as follows
(Fig. 66).
The first and second arches are completely obliterated.
The third arch on each side is continued at its distal end as
the internal carotid, J.(7a, the connection between it and
the fourth arch having become entirely obliterated. From
its proximal end as the direct continuation of the trunk which
originally supplied the first and second arches the external
carotid, E.CA., is given off. Each pair of carotids arises
therefore from a common trunk the common carotid (C.
G.A.}. Each of these trunks gives off near its proximal
end a branch, the vertebral artery (V.A.), which joins at
its distal end a branch from the external carotid.
The common carotid on the right side comes off from the
fourth arch of the right side (the arch of the dorsal aorta),
and is not as yet connected with the right subclavian, R.sc.
The common carotid of the left side comes off from the
fourth arch of the left side ; but since this arch becomes the
left subclavian, L.sc. (the connection between the fourth and
fifth left arches being obliterated), the portion of the trunk
(L.I.N.) between the fourth arch and the bulbus arteriosus (or
as it must now be called the common aortic root) is called
the left innominate artery.
The fourth arch of the right side forms the commence-
ment of the great dorsal aorta, and gives off the right
subclavian (R. sc.) just before it is joined by the fifth arch.
The fifth arch of each side gives off branches P.A. to the
lungs ; their distal continuations RC.b., L. C.b. y by which they
are connected with the systemic circulation, though much
reduced, are not obliterated.
The final changes undergone by the arterial system
after the commencement of the pulmonary respiration consist
VIII.]
THE DUCTUS BOTALLI.
219
chiefly in the complete separation of the pulmonary and
systemic circulations.
RIN
AO
DlAGKAM OF THE ABTEKIAL SYSTEM OF THE ADULT FOWL.
R.I.N. right innominate artery. The other letters as in Fig. 66.
The dotted lines, as before, shew the portions of arches which have been
obliterated.
As the branches to the lungs become stronger and
stronger, less and less blood from the right ventricle enters
into the dorsal aorta; and the connecting vessels become
smaller and smaller.
Each of the arches from the right ventricle may therefore
be considered at about the sixteenth or eighteenth day as
divided into two parts, an inner part which connects the heart
with the lung, and an outer part which still connects the arch '
with the main dorsal aorta. As these outer parts become
smaller they receive the name of the 'ductus or canales
Botalli' or 'ductus arteriosi.' The one on the right side is
short ; that on the left side is much longer and narrower.
Von Baer supposed that the reason of this was, that since the pulmonary
arch of the left side was the fourth, the ductus Botalli of that side consisted of
the branch between the fourth and fifth arch, as well as between the fifth arch and
the dorsal aorta. It is easy however from diagram (Fig. 66) to see that this
220 THE SIXTH DAY. [CHAP.
reason is superfluous, and that the explanation is that the canalis Botalli of the
right side is merely the portion of the fifth arch between the origin of the
pulmonary artery and the junction of the fifth arch with that common trunk,
with which all the arches of that side are at one time or other connected,
and which remains as the continuation of the aortic arch into the dorsal aorta;
while on the left side it consists of the same portion of the arch as on the right
side, and also of the corresponding common trunk of the left side.
When respiration commences the blood ceases to pass
through these canals, which either remain as mere ligaments
or else become absorbed altogether. By this means, the
foramen ovale becoming at the same time closed, a complete
double circulation is established. All the blood from the
right ventricle passes into the lungs, and all that from the
left ventricle into the body at large.
Two other changes take place about the same time in
the aortic branches. That portion of the right fourth or
aortic arch which lies between the origin of the right sub-
clavian and the common carotid becomes shortened, and
is finally swallowed up in such a fashion that the right
subclavian (Fig. 67, R.sc.) comes off from the right common
carotid, a very short trunk being left between the union of
the two to serve as the right innominate artery.
At the same time, corresponding to the increase in the
length of the neck, the common carotids are very greatly
lengthened. They lie close together in the neck, and in
many birds actually unite to form a common trunk.
It will of course be understood that with the disap-
pearance of the allantois and the absorption of the yolk,
the umbilical and omphalo-mesaraic arteries also disappear.
11. It may perhaps be of advantage to the reader if we
here briefly summarize the condition of the circulation at
its four most important epochs; viz. on the third day, on
the fifth day, during the later days of incubation before
respiration by the lungs has commenced, and after the chick
has begun to breathe by the lungs.
On the third day the circulation is of an exceedingly
simple character.
The heart is to all intents and purposes a simple twisted
tube marked off by constrictions into a series of three con-
secutive chambers. The blood coming from the venous
radicles passes through the heart and then through the
three pair of arterial arches.
VIII.] THE CIRCULATION ON THE FIFTH DAY. 221
From these it is collected into the great dorsal aorta.
On this dividing into two branches, the stream of blood also
divides and passes down on each side of the notochord along
the body, and thence out by the omphalo-mesaraic arteries,
which distribute it to the yolk-sac.
In the yolk-sac it partly passes into the sinus terminalis
and so into the fore and aft trunks, partly directly into the
lateral trunks, of the omphalo-mesaraic veins. In both cases
it is brought back to the two venous radicles and so to the
heart.
On this day the blood is aerated in the capillaries of the
yolk-sac.
On the fifth or sixth day, the two auricles are present
though having a common cavity. The septum of the ven-
tricles is nearly complete, so that the blood on entering the
ventricles from the auricles is divided into two streams.
These two streams pass respectively from the right and left
chambers of the heart into the two divisions of the bulbus
arteriosus. The blood from the right ventricle passes into
the fifth pair of arches and that from the left ventricle into
the third and fourth pairs of arches.
From the anterior parts the blood is brought back by the
anterior cardinal or jugular veins ; from the hinder parts of
the body, chiefly by the cardinal veins, but also in part by
the now commencing vena cava inferior.
The blood from the yolk-sac and allantois, together with
a small quantity from the intestine, is collected into the
portal vein, and by that vessel carried to the liver. Here it
becomes divided into two streams, part flowing directly by
the ductus venosus into the sinus venosus, and the remainder
passing through the capillaries of the liver and being brought
back to the sinus venosus by the hepatic veins.
During this period the blood is aerated both by the
allantois and yolk-sac, but as yet chiefly by the latter.
At a somewhat late period of incubation, the blood from
the ventricles passes into two entirely distinct roots. The
one of these, that from the right chamber, sends the blood to
the fifth pair of arches. Passing through these two arches,
the greater part of the blood flows into the dorsal aorta, a
small portion only finding its way into the lungs through the
as yet unimportant pulmonary arteries.
222 THE SIXTH DAY. [CHAP.
Through the other aortic root, viz. that from the left
ventricle, the blood flows into the third and fourth pairs of
arches. That part of the blood which flows into the third
pair of arches, passes almost entirely to the head and upper
extremities by the external and internal carotids ; that which
flows into the right arch of the fourth pair, is chiefly brought
to the dorsal aorta, but some of it passes to the right wing ;
that, on the contrary, which goes into the left fourth arch, is
for the most part sent to the wings, a small part only reaching
the dorsal aorta. There is still a mixture of the blood from
the two chambers of the heart, so that the blood in the
dorsal aorta is composed partly of the blood from the Ieft 5
and partly from the right chambers. The blood of the upper
(anterior) end of the body comes entirely from the left ventricle.
The blood of the dorsal aorta passes to the yolk-sac and
allantois, and to all the hinder parts of the body. It
is brought back from the yolk-sac, from the allantois, and
to a certain extent from the intestines, by the portal vein,
part of the blood from which passes to the inferior vena cava
by the direct course (ductus venosus), and part indirectly by
the more circuitous course of the capillaries of the liver and
hepatic veins.
The blood from the generative and urinary organs, and
from the hinder extremities, is brought back to the heart by
the vena cava inferior ; that from the upper extremities and
head by the jugular, vertebral and wing veins into the two
venae cavae of the right and left side, and so to the heart.
Of these three venae cavae, the left superior and the inferior
open into the heart independently. The right superior enters
with the inferior. All of these enter the right auricle, but
the common opening of the inferior and right superior venae
cavae is so directed, that the blood carried by those vessels
flows chiefly through the foramen ovale into the left auricle.
The blood from the left superior vena cava enters the right
auricle only. Now the blood of the inferior vena cava
has been partly aerated by the allantois ; and, since it is this
blood which passing through the left auricle and ventricle is
distributed to the third aortic arch, unmixed by any blood
from the right ventricle (the mixture with the blood from the
fifth arch taking place in the fourth arch only), it happens
that the blood which flows to the anterior extremities and
VIII.] THE ADULT CIRCULATION. 223
head, is more aerated than that in any other part of the
body.
From the anterior extremities the blood is to a great
extent returned by the left superior cava, and goes into the
right auricle, whence, by the right ventricle, it is distributed
through the fifth pair of arches over the body, after joining
the more aerated blood passing through the fourth pair of
arches.
The blood from the lungs is brought back by two small
veins into the left auricle.
The characteristics of the circulation at this time, are that
the blood is aerated by the allantois, and that there is a
partial double circulation. (Vide Chap. VII. 9.)
As soon as respiration commences, the canals leading to
the dorsal aorta from the fifth pair of arches, which com-
municate only with the right ventricle, become closed. The
blood passing along the fifth arch now flows only into the
lungs, through the pulmonary arteries. The blood from the
left ventricle owing to the cessation of the circulation of the
yolk-sac and the allantois is distributed exclusively to the
body of the chick, from whence it is all brought back into
the right auricle by the three now independently opening
venae cavse.
The portal veins henceforward receive blood from the intes-
tines only, and the ductus venosus is obliterated, so that
all the blood of the portal vein passes through the capil-
laries of the liver.
The partition between the auricles is rendered complete
by the closure of the foramen ovale ; into the right auricle
the veins of the body enter, and into the left the pulmonary
veins.
There is thus a completely double circulation formed, in
which all the blood of the left ventricle is arterial, and all the
blood of the right ventricle venous, and there is at no part
of the circulation a mixture of venous and arterial blood.
12. As early as the sixth day, movements, as we have
said, may be seen in the limbs of the embryo upon opening
the egg. We may conclude that after this epoch spontaneous
movements occur from time to time in the unopened egg.
They cannot however be of any great extent until the four-
teenth day, for up to this time the embryo retains the position
224 THE SIXTH DAY. [CHAP. VIII.
in which it was first formed, viz. with its body at right angles
to the long axis of the egg.
On the fourteenth day a definite change of position takes
place; the chick moves so as to lie lengthways in the egg,
with its beak touching the chorion and shell membrane where
they form the inner wall of the rapidly increasing air-
chamber at the broad end (Chap. I. 2).
On the twentieth day or thereabouts, the beak is thrust
through these membranes, and the bird begins to breathe the
air contained in the chamber. Thereupon the pulmonary
circulation becomes functionally active, and at the same time
blood ceases to flow through the umbilical arteries. The
allantois shrivels up, the umbilicus becomes completely
closed, and the chick piercing the shell at the broad end of
the egg with repeated blows of its beak, casts off the dried
remains of allantois, amnion and chorion, and steps out into
the world.
CHAPTER IX.
THE DEVELOPMENT OF THE SKULL.
1. IN the chapter on the fifth day, we gave a short
description of the earliest stages of the development of the
skull. The subject is however of sufficient importance to
merit a separate chapter, and in order to render the present
account complete in itself, we have found it necessary to
repeat a few of the statements already made.
2. In its earliest condition the cranium is composed of a
mesoblastic tissue of stellate cells which can be distinguished
from the remainder of the mesoblast by its greater opacity.
In this condition (which is that of the fourth day), it may be
spoken of as the membranous cranium. From this mem-
branous condition the tissue composing it rapidly passes into
true hyaline cartilage.
3. The primitive skull 1 , from its very first formation on
the fourth day, consists of elements which fall into two very
distinct divisions. We have on the one hand a sheet of
cartilage which ensheaths the notochord from its anterior
end to the first vertebra. This sheet of cartilage forms an
unsegmented continuation of the vertebral bodies. It is to
be considered as the most anterior portion of the axial
skeleton, in which the segmentation has become obliterated ;
j and as such is equivalent not to one, but to a (hitherto not
certainly determined) number of vertebrae.
This sheet was spoken of by Rathke (Entwickelungs-
geschichte der Natter), its discoverer, as the 'investing mass'
1 The facts narrated in this chapter are mainly derived from Mr Parker's
I Memoir upon the Development of the Skull of the Common Fowl (Gallus
k domesticus), Phil. Trans., 1866, Vol. CLVI., pt. i.
E. 15
226 THE DEVELOPMENT OF THE SKULL. [CHAP.
from its relations to the notochord, and as such we shall
continue to speak of ity
The second of the two divisions into which the parts of
the skull fall, consists of a series of paired rods, whose
proximal extremities! are attached more or less closely to the
investing mass. All of these (with the exception of the
trabeculse) are developed along the axes of the visceral
arches.
FIG. 68.
*-cv 1
VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL^ ON
THE FOURTH DAY OF INCUBATION. (From Parker.)
In order to shew this the whole of the upper portion of the head has been
sliced away. The cartilaginous portions of the skull are marked with the dark
horizontal shading.
cv I cerebral vesicles (sliced off), e eye. nc notochord. iv investing mass.
9 foramen for the exit of the ninth nerve, cl cochlea, ksc horizontal
semicircular canal, q quadrate. 5 notch for the passage of the fifth nerve.
lg expanded anterior end of the investing mass, pts pituitary space.
tr trabeculse. The reference line tr has accidentally been made to end a
little short of the cartilage.
We shall commence by describing the ' investing mass *
as we find it on the fourth day, and from this pass on to
the paired rods attached to it.
IX.] THE INVESTING MASS. 227
4. The investing mass (Fig. 68, iv) is on the fourth day
a broad plate of tissue ensheathing the notochord and arching
upwards on each side and especially behind. Laterally it
encloses the auditory sacs, and the tissue surrounding these
(forming the so-called 'periotic capsules') is in the chick never
separate from the investing mass. In front it becomes
narrowed, and at the same time excavated so as to form
a notch on each side (Fig. 68, 5) through which the fifth
nerve passes ; and in front of this again it becomes expanded.
In order to render our subsequent account more intelligi-
ble, we may shortly anticipate the fate of the investing mass.
Behind it grows upwards, and the two outgrowths meet
above so as completely to enclose the medulla oblongata,
and to circumscribe a hole known as the 'occipital foramen'
And it is at this point only that the roof of the skull is at
any period formed of cartilage.
Later than this there appear in the investing mass a
series of ossifications forming the whole of the occipital bone,
and the skeleton of the ear. Knowing this, we shall be able
to speak of the different parts of the investing mass, as the
regions of the various bones ' ex-occipital/ f basi-occipital' etc.
even before the ossifications, which mark these out, have
appeared.
5. In considering the paired rods of cartilage attached to
the investing mass, we will commence with the most an-
terior. These are known as the 'trabeculce cranii (Fig. 68, tr).
They are two narrow rods whose proximal ends are attached
to the front end of the investing mass, of which they appear
like a forward continuation, while distally they unite with
each other. There is thus left an oval (or more nearly
circular) space where the cartilage forming the base of the
skull is deficient. This space is enclosed behind by the in-
vesting mass, and on the two sides and in front by the trabe-
cute; it is called the 'pituitary space' (Fig. 68, pts), and in
it lies the 'pituitary body/
Where the trabeculae unite in front, they form a some-
what expanded plate of cartilage continued anteriorly into
two diverging horns, which subsequently develope into the
all-nasal cartilages. Owing to the cranial flexure the trabe-
culge at first lie in a different plane from the investing mass.
In function, the trabeculse seem almost to serve as a con-
152
228 THE DEVELOPMENT OF THE SKULL. [CHAP.
tinuation of the base of the skull, and such they were con-
sidered to be by Rathke, their discoverer. Their different
mode of development, and (in the lower vertebrates) primitive
independence of the investing mass, prove that this is not the
case, and that they are almost undoubtedly to be looked on
as paired appendages. Their primitive independence of the
investing mass is clearly shewn in the skulls of the Marsipo-
branchii, in which the trabecute are formed of a dense
fibrous tissue and the investing mass of hyaline cartilage.
(W. Muller, Ban der Hypophysis u. des Processus infundibuli
cerebri. Jenaische Zeitschrift, Vol. vi.) A very probable
view, first put forward by Huxley in his Hunterian lectures
(vide Anat. Vertebrates, p. 77), and subsequently adopted by
Parker as well as other investigators, is, that they are homo-
logous with the rods of cartilage developed in the visceral
arches, and that they are therefore the remnants of an
anterior pair of arches.
Gegenbaur (Vergleichende Anatomic der Wirbelthiere, in. Heft) looks upon
the trabeculse and their coalesced extremities simply as a prevertebral portion
of the cranium, and not as paired appendages of the investing mass. Their
paired condition seems to militate against this view ; it must be admitted how-
ever that our present knowledge of them does not permit us to state with
certainty more than that they are not morphologically a continuation of the
investing mass.
In the bird and apparently many higher vertebrates they
are never independent of the investing mass, and it was this
fact which led to the erroneous view of their nature, formerly
adopted, that they were forward continuations of the axial
skeleton.
In them and in the plate of cartilage formed by their
coalescence in front appear the ossifications of the whole of
the sphenoid, the ethmoid and nasal regions.
6. The remainder of the paired series of appendages
are developed in the visceral arches. The foremost of
these are the cartilaginous rods developed in the first visceral
or mandibular arch. In our account of the face we men-
tion that the mandibular arch on each side produces a
bud known as the superior maxillary process, which goes
to form the superior boundary of the mouth. In this
process, as well as in the primitive arch, rods of cartilage
appear.
The segmentation, so to speak, of the first visceral arch
IX.] THE MAXILL1KY PROCESS. 229
appears when taken by itself somewhat remarkable, and led
to the view, formerly very general and still held by many,
that the superior maxillary process was equivalent to a
visceral arch. Mr Parker has recently shewn that in both
Osseous fishes and the Elasmobranchii all the anterior arches
undergo a somewhat similar segmentation, and it appears
therefore to be a fair deduction that this segmentation
persists in the chick in the mandibular arch, while it has
been lost in the others. None of Mr Parker's investigations
on the lower vertebrates support the view that the maxillary
process is an aborted anterior arch. Similar views as to the
nature of the maxillary process have been arrived at by
Gegenbaur (loc. cit.}, from his investigations upon the crania
of the cartilaginous Fishes.
7. The cartilage or differentiated mesoblast appears in
the maxillary process later than in the mandibular arch,
FIG. 69.
VIEW FBOM BELOW OP THE PAIRED APPENDAGES OF THE SKULL OP A FOWL
ON THE FOURTH DAT OP INCUBATION. (From Parker.)
cv i cerebral vesicles, e eye. fn fron to-nasal process, n nasal pit. r trabeculae.
pts pituitary space, mr superior maxillary process, pg pterygoid. pa pala-
tine, q quadrate, mk Meckel's cartilage, ch cerato-hyal. bh basi-hyal.
cbr ceratobranchial. ebr proximal portion of the cartilage in the third
visceral arch. 66r. basibranchial. i first visceral cleft. 2 second visceral cleft,
3 third visceral arch.
230 THE DEVELOPMENT OF THE SKULL. [CHAP.
and consists of a rod on each side. Each of these early
becomes divided into two parts, a proximal and a distal.
From the bones which subsequently develope in them the
proximal one is known as the pterygoid (Fig. 69, pg\ and
the distal one as the palatine (Fig. 69, pa). Both of the
rods are very delicate, and remain in the cartilaginous con-
dition for a short time only.
8. In the mandibular arch itself, there is also a proxi-
mal and distal cartilage. The proximal cartilage is situated
(Figs. 68 and 69, q) at the side of the investing mass but
not united with it. It is known as the quadrate, and in the
early stage is merely a small knob of cartilage. The distal
rod is called MeckeTs cartilage (Fig. 69, mfy ; on it are sub-
sequently moulded the bones which form the mandible;
while its proximal end becomes the articulare.
9. In the next arch, usually called the second visceral
or hyoid arch, there is a very small development of cartilage.
This consists of a central azygos piece the 'basi-hyaV (Fig. 69,
bh), and two rods, one on each side, the ' cerato-hyals' (Fig. 69,
ch).
10. In the third arch, which corresponds with the first
branchial arch of the Ichthyopsida, there is on each side a
large distal cartilaginous rod (Fig. 69, cbr) the ' cerato-
branchiall and a smaller proximal piece (Fig. 69, ebr) ; be-
tween the two arches lies an undefined mass (Fig. 69, bbr)
the ' basibranchial? In the arches behind this one there is
in the bird no development of cartilage.
11. The growth of this primordial cartilaginous skull is
very rapid, and by the 5th or 6th day very important changes
have taken place. In the first place the pre-pituitary portion
of the cranio-facial axis becomes equal to and subsequently
actually surpasses in length the post-pituitary part. This
change is accompanied by a considerable decrease in the
cranial flexure. In the investing mass, the chief changes
are an upward growth on each side behind, to form the exoc-
cipitals ; and the appearance of the occipital condyles (Fig.
70, oc) as small swellings on each side of the middle line
at the hind end of the plate. In front of the termination
of the notochord two transverse vertical walls of cartilage
rise up, the one in front of and the other behind the pitui-
tary body: these are called respectively the anterior and
IX.] THE ETHMO-PRESPHENOID PLATE. 231
posterior clinoid walls. But it is in front of the foremost of
these that the most noticeable changes take place. From
the mid-line of the coalesced trabeculsB there rises up a
high ridge, the ethmo-presphenoid plate. This plate is at first
highest behind. In front and below it sends out a process,
the prenasal cartilage, which forms the cartilaginous basis
on which the premaxillary region is moulded.
12. Development continues to be very rapid in these
parts; and on the seventh day the anterior end of the
ethmo-presphenoid plate (Fig. 70, eth and ps) becomes its
highest point and forms the retral spike of the ethmoid
(Fig. 70, eth).
FIG. 70.
bbr
SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THE SEVENTH
DAY OF INCUBATION. (From Parker.)
pn prenasal cartilage, din alinasal cartilage, ale aliethmoid ; immediately
below this is the aliseptal cartilage, eth ethmoid, pp pars plana. ps pre-
sphenoid. pa palatine, pg pterygoid. , z optic nerve, as alisphenoid.
q quadrate, st stapes, fr fenestra rotundum. hso horizontal semicircular
canal, psc posterior vertical semicircular canal : both the anterior and the
posterior semicircular canals are seen shining through the cartilage,
so supraoccipital. eo exoccipital. oc occipital condyle. nc notochord.
mk Meckel's cartilage, ch cerato-hyal. bh basi-hyal. cbr and ebr cerato-
branchial. bbr basibranchial.
The prenasal cartilage (Fig. 70, pn) still points down-
wards, and by this time are formed the alinasal cartilages
(Fig. 70, alri) developed from the trabecular horns, and th
aliseptal cartilages which enclose the inferior turbinals
(Fig. 70).
The basisphenoid grows outwards on each side to form
232 THE DEVELOPMENT OF THE SKULL. [CHAP.
the alisphenoid (Fig. 70, as) and posteriorly the supraoccipitals
(Fig. 70, so) have approached much nearer to each other above ;
and at the same time the thickenings to form the occipital
condyles have greatly increased (Fig. 70, oc).
The quadrate (Fig. 70, q) has undergone great modifica-
tions. In the earlier stage it was a simple knob of cartilage,
but now it sends a process forwards, the orbital process, and
a long process backwards which articulates with both the
'periotic capsule' and the exoccipital (Fig. 70, eo)\ down-
wards it sends a two-headed process to articulate with the
extremity of Meckel's cartilage.
The fenestra ovale and fenestra rotundum (Fig. 70, fr)
appear during this stage, and in the former the head of the
small stapes (Fig. 70, st) is placed.
The palatine (Fig. 70, pa) and pterygoid (Fig. 70, pg)
bars have increased in length, the former being the longer;
between them and the base of the skull there has appeared
a mass of tissue which will eventually become the sphenoidal
rostrum (parasphenoid).
The second arch is not changed much; while the parts
of the third arch, corresponding with the first branchial arch
of osseous fishes, have increased in size, but have not under-
gone other modifications.
13. In the next (Parker's third) stage, which occurs
about the middle of the 2nd week of incubation, the pre-
nasal cartilage (Fig. 71, pri) has greatly increased in length
and lost its downward curvature, so that it is in the same
straight line as the septum nasi and the ethmoid; the con-
tinuous plate thus formed while increasing in length is at
the same time narrowed except just in front of the pitui-
tary space, where it becomes expanded on each side into an
ear-shaped process.
The pituitary space is still open and admits the internal
carotids; behind it is another azygos slit (Fig. 71, nc) along
the median line, in which the notochord can be seen. This slit
is a new formation, and as the skull outgrows the notochord,
the surrounding parts will become the post-pituitary part of
the basisphenoid.
The outgrowths forming the alisphenoids have largely
increased. When looked at from below they are almost
entirely concealed by the ossified basitemporals (Fig. 71, bf) t
IX.] THE OSSIFICATIONS OF THE CRANIUM. 233
but their anterior corners just project out in front of it
(Fig. 71, CM). ^
The occipital condyle formed by the coalescence of the
two originally separate cartilaginous knobs, is seen just below
the notochord (Fig. 71, no), and the supraoccipitals (Fig.
71, so) have coalesced above the occipital foramen.
14. The ossifications of the cartilaginous skull, which
have commenced at this stage, are 1 :
(1) An ectosteal ossification around the notochord on
the inner surface of the skull immediately behind the
occipital condyle. This soon spreads to the opposite surface
and forms an ossifying centre for the basioccipital.
(2) An ectosteal ossification (Fig. 71, eo) in each exoc-
cipital beginning immediately behind the vagus foramen
(Fig. 71, 8) ; this ossification commences on the exterior, but
soon spreads round to the inner surface of the cartilaginous
exoccipital.
(3) The palatine (Fig. 71, pa) has become entirely
ossified by endostosis, the only instance of this process in the
ossification of the primordial skull of the bird.
(4) An ectostosis of the pterygoid (Fig. 71, pg)*
(5) An ectosteal ossification of the quadrate (Fig. 71, q).
(6) An ectosteal ossification of the cerato-branchials.
15. At this stage the majority of the true membrane
bones also begin to be formed.
The paired premaxillaries (Fig. 71, px) are formed in
the investment of the prenasal cartilage ; they are trian-
gular with the apex directed forwards, and soon begin to
develope their three normal processes : one above towards
the frontal (the nasal process), one along the margin of the
beak to join the maxillary (the marginal process), and one
below along the middle of the palate, to join the palatine
bones. The maxillaries (Fig. 71, mx) are developed on
each side outside the endoskeleton. Pointed at each end,
they are broader in the middle, sending a process inwards
1 The term ectostosis or ectosteal is used when ossification sets in between the
perichondrium and the cartilage, endostosis when ossification takes place be-
tween the actual cells of the cartilage. These two processes not unfrequently
occur combined, when ossification by ectostosis on reaching the cartilage sets
up a true endosteal ossification. Parostosis is used for all ossifications which
take place in purely fibrous tracts. Parosteal products are often spoken of
as membrane or splint bones.
234
THE DEVELOPMENT OF THE SKULL.
FIG. 71.
[CHAP.
rbs
EMBRYONIC SKULL OP A FOWL DURING THE SECOND WEEK OP INCUBATION
(THIRD STAGE) FROM BELOW. (From Parker. )
The figure is intended to shew the cartilaginous skull with its ossifications,
and also the splint bones which have commenced to be formed independently of
the cartilaginous cranium. The parts of the lower jaw and hyoid are omitted.
The following letters refer to the cartilaginous cranium and its ossifications.
pn prenasal cartilage, pa palatine bone (which has already become ossified).
pg pterygoid (also ossified), nc notochord visible in the post-pituitary space.
Both the notochord and the post-pituibary space would be in reality per-
fectly concealed by the plate of bone bt, but have been diagrammatically re-
presented as if they were visible through it. nc end of notochord appearing
beyond the occipital condyle. q quadrate, as alisphenoid. ic internal
carotid, ty tympanic cavity, psc posterior semicircular canal. 8 foramen for
passage of eighth nerve. 9 foramen for passage of ninth nerve, eo centre of
ossification for the occipital, so supraoccipital region, fm foramen magnum.
The following letters refer to the splint bones.
px premaxillary. mx maxillary, pmx process inwards from the premaxillary to
form the maxillo-palatine. j jugal. qj quadrato-jugal. rbs sphenoidal
rostrum, bt basitemporal. '.,;
IX.] THE PARASPHENOID. 235
to form the maxillo-palatine (Fig. 71, pmx). The jugal
(Fig. 71, j) and the quadrato-jugals (Fig. 71, qj) are also
developed during this stage, and connect the hinder end of
the maxillaries with the quadrate ; they are both delicate
rod-like bones, the quadrato-jugal being the larger.
The top of the cranium, which up to this* time has not
been covered by either cartilage or bone, begins now to be
closed in by parosteal osseous deposits taking place to form
the nasals, frontals, and parietals. Outside the prootic
region the squamosal also begins to be formed by a deposit
taking place in the mesoblast external to the perichondrium.
In the base of the skull, there are three osseous deposits
to form membrane bones, which soon uniting constitute the
equivalent of the parasphenoid of osseous fishes.
The first of these is the osseous deposit which takes place
in the mass of tissue, previously spoken of as the rostrum
(Fig. 71, rbs); this is connected behind with two osseous
deposits, the basisphenoidal ossicles, formed one on each side
in front of the pituitary space. The third deposit is the
large somewhat reniform mass of bone (Fig. 71, bt) called
the basitemporal, which forms one of the most conspicuous
features in the skull at this stage, and consists of two sym-
metrical halves. The basitemporals have not at this stage a
very firm connection with the base of the skull, but shortly
they become firmly united with it and with one another, and
thus the pituitary space becomes entirely closed over below
by bone.
16. During the previous stage, most of the bones of the
face and skull had begun to be ossified, so that during the
next or fourth stage, that is about the end of the second and
the beginning of the third week of incubation, the chief
changes consist in the progressive ossification of the various
bones. Since it is not our purpose to enter fully into the
structure of the skull of the fowl but rather to confine our-
selves to its early development, this and the succeeding
stages will be described with greater brevity than the first
three.
17. One most important change which occurs during this
stage, is the fenestration of the ethmo-presphenoid cartilage.
This cartilage (Fig. 70, ps) formed during the second stage
an unbroken plate, but towards the end of the third and
236 THE DEVELOPMENT OF THE SKULL. [CHAP.
the beginning of the fourth stages, two fenestrae appear.
The hindermost of these separates the presphenoid from the
ethmoid ; but since the hole does not extend to the bottom
of the plate, the basi-sphenoid and the ethmoid are continu-
ous below. The other fenestra separates the ethmoid from
the nasal septum.
The early condition of the ethmoido-nasal cartilage re-
sembles, as Parker points out, its permanent condition in the
Struthiadce, while the condition which it acquires during this
stage resembles its condition in Tinamine birds, seeming to
prove that the Gallinaceous birds passed through these two
stages before reaching the present condition. This plate of
cartilage remains during this stage quite unossified. The
aliethmoid, aliseptal, and alinasal cartilages which grow out
from it on each side, are now severally separated from each
other by slight grooves.
18. The chief new ossifying centres which appear in the
primordial skull in this stage are :
(1) An ectosteal plate for the prootic.
(2) Two ectosteal plates which appear in each ali-
sphenoid, one in the upper corner, and another immediately
above the foramen ovale.
The centres which had begun during the last stages have
now spread considerably.
The basioccipital is to a great extent ossified, but still
encloses the remains of the notochord. The occipital condyle
is however still unossified. Both the exoccipital and supra-
occipital plates are spreading rapidly, and the latter have
nearly reached the middle line. The quadrate is nearly
entirely ossified, but its upper and lower condyles and orbital
process are still cartilaginous.
19. The rostrum is still separate from the ethmoid,
but has to a great extent coalesced with the basisphe-
noid. Between the posterior end of the pterygoid and the
rostrum, a plate of cartilage is interposed, called the basi-
pterygoid.
The basitemporals have coalesced to a great extent with
the osseous outgrowths of the basisphenoid, but at the sides
a space is left between the two, through which the Eusta-
chian tubes pass. Between the basitemporals themselves,
where they have not coalesced, another space is left, which
IX.] THE OSSIFICATIONS OF THE EAR. 237
becomes part of the common vestibule in which the Eusta-
chian tubes meet in the middle line.
One new splint bone, the vomer } begins to be formed
during this stage in the middle line about half way along
the palatines.
The various membrane-bones of the skull are now much
more firm and consistent, and the frontals have sent down a
process which upon reaching the ethmoidal plate at once
sets up ossification in it.
20. In the next stage, about the second day after birth,
a considerable number of changes have taken place. The
ethmoid has begun to be ossified by two ectosteal plates, one
on each side, and the ossification set up by the frontal process
in the ethmoid plate has increased. The ethmoid is now only
connected with the septum nasi by a narrow isthmus above.
The septum still retains its anterior process, the pre-
nasal cartilage, but this is rapidly diminishing.
In the ear there are now three ectosteal ossifications, the
prootic, which appeared in the last stage, and which is by far
the largest of the three ; the opisthotic, which lies between
the prootic and the exoccipital, but is distinct from both ; and
a third small ectosteal plate, the pterotic.
A small ring-like ossification appears in the internal
angular process of Meckel's cartilage ; its other process
rapidly diminishes and soon disappears.
The supraoccipital ossifications have united in the
middle line ; the exoccipitals have increased very much, but
are quite separate from the opisthotics.
The basioccipital ossification has begun to spread into
the condyles.
21. The chief facts of importance in reference to the
splint bones, are, that the premaxillaries (Fig. 71, px) have
united in the middle line ; the vomer has considerably
increased in size; the lacrymals have developed, and
possess a large supraorbital and a thick anteorbital plate.
There is still a large fontanelle above, between the frontals
and parietals. The squamosals are very large. In the man-
dible all the splint bones have by this time become developed.
At about the third week after birth two new centres
appear, one at the outside of the alisphenoid, forming the
centre for the post-frontal, and another over the posterior
238 THE DEVELOPMENT OF THE SKULL. [CHAP. IX.
semicircular canal, for the epiotic, which is for a short time
distinct from the exoccipital.
In chicks of about two months old, the sutures have begun
to lose their distinctness, but two new pairs of centres of
ossification appear adjacent to the presphenoid. These first
ossify independently, but afterwards set up ossification in the
presphenoid, which is still unossified. In fowls of from seven to
nine months, the splint bones are beginning to coalesce for the
most part with the other bones of the skull, and the sutures
are rapidly being obliterated. It would be beyond the scope
of this work to enter into the changes by which the skull of
the young chick, with most of the sutures distinct, developes
into the skull of the adult, where the lines of junction of
most of the bones are quite undistinguishable. We shall
therefore conclude by giving a table of those bones which
are preformed in cartilage, and of the purely splint or
membrane bones.
Parts of the birds skull which are either preformed in
cartilage or remain cartilaginous.
Supraoccipital. Exoccipital. Basioccipital. Epiotic.
Prootic. Opisthotic. Pterotic. Alisphenoid. Basisphenoid.
Orbitosphenoid. Presphenoid. Ethmoid. Post -frontal.
Septum nasi, turbinals, prenasal and nasal cartilages. Skele-
ton of the second and third visceral arches and stapes.
Meckel's cartilage and quadrate (first visceral arch). Ptery-
goid and palatine (superior maxillary process).
Splint-bones not preformed in cartilage.
Parietals. Squamosals. Frontals. Lacrymals. Nasals.
Premaxillse. Maxillae. Maxillo-palatines. Vomer. Jugals.
Quadrato-jugals. Dentary and Bones of Mandible. Basi-
temporal and rostrum.
APPENDIX.
PRACTICAL INSTRUCTIONS FOR STUDYING THE DEVELOPMENT
OF THE CHICK.
I. Incubators.
OF all incubators, the natural one, i.e. the hen, is by far
the best. The number of eggs which fail to develope is much
fewer than with an artificial incubator, and the progress of
development is much more regular. Under the hen an egg
taken, say at 36 hours, will most probably be found in the
stage which we have described as proper to that date ; in an
artificial incubator, it is nearly sure not to be so. A good
sitter will continue to sit for thirty or more days at least,
even though the eggs are daily being changed. She should
never be allowed to want for water, and should be well
supplied according to her appetite with soft food. It is best
to place the food at some little distance from the eggs, in
order that the hen may leave the eggs when feeding. She
will sit most persistently in a warm, quiet, somewhat darkened
spot. When an egg is placed under her, the date should be
marked on it, in order that the duration of its incubation
may be exactly known. When the egg is intended to remain
for some time, e.g. for seven days or more, the mark should
be bold and distinct, otherwise it will, be rubbed off.
Where a "broody" hen cannot be obtained, recourse must
be had to an artificial incubator. We have ourselves been
so accustomed to employ a hen, that we have very little
experience of the various machines which have been intro-
duced as incubators. We have, however, obtained tolerably
satisfactory results with an ordinary chemical double-jacketed
drying water-bath, thoroughly covered in with a thick coat of
ootton wool and flannel baize, and heated by a very small
240 PRACTICAL DIRECTIONS. [APP.
gas-jet. If the vessel be filled with hot water, and allowed to
cool down to 40 or thereabouts, before the eggs are intro-
duced, a very small gas flame will be sufficient to maintain
the requisite temperature. A small pin-hole-nozzle, giving
with ordinary pressure an exceedingly narrow jet of flame
about two inches high, is the most convenient. By turning
the gas off or on, so as to reduce or increase the height of the
jet as required, a very steady mean temperature may be
maintained. A simple arrangement of this kind on the whole
works more satisfactorily than any of the complicated instru-
ments which have been introduced for similar purposes.
In the absence of gas, a patent night-light placed at a
proper distance below the bath may be made to answer very
well. When a body of water, once raised to the necessary tem-
perature, is thoroughly surrounded with non-conducting ma-
terial, a very slight constant amount of heat will supply all
the loss.
The temperature should be from 37 to 40C. ; it may rise
temporarily a few degrees above 40 without any permanent
harm, and should not be allowed to fall below 37.
The eggs within the bath should be placed on and covered
up with cotton wool ; and the products of the combustion of
the gas should be kept as much from them as possible.
II. Examination of a 36 to 48 hours embryo.
The student will find it by far the best plan to begin
with the study of an embryo of this date. The manipulation
is not difficult ; and the details of structure are sufficiently
simple to allow them to be readily grasped. Earlier embryos
are troublesome to manage until some experience has been
gained ; and the details of later ones are so many as to render
it undesirable to begin with them.
A. Opening the egg.
Take the egg warm from the hen or the incubator,
and place it (it does not matter in what position,
since the blastoderm will at this stage always be found
at the uppermost part of the egg) in a small basin
large enough to allow the egg to be covered with
fluid. It is of advantage, but not necessary, to place
PP.] OPENING THE EGG. 241,
at the bottom of the basin a mould, e.g. a flat piece of
lead with a concavity on the upper surface, in which
the egg may rest securely without rolling. Pour into
the basin so much of a "75 per cent, solution of sodium
chloride warmed to 38 as will cover the egg com-
pletely. With a sharp tap break through the shell at
the broad end over the air-chamber, and let out as
much air as has already been gathered there. Unless
*: this is done, the presence of air in the air-chamber
will cause the broad end to tilt up. At this date
there will be very little air, but in eggs of longer
incubation, inconvenience will be felt unless this plan
be adopted.
Instead of being broken with a blow, the shell
may be filed through at one point, and the opening
enlarged with the forceps ; but a little practice will
enable the student to use the former and easier
method without doing damage.
With a blunt pair of forceps, remove the shell
carefully bit by bit, leaving the shell-membrane
behind ; begin at the hole made at the broad end, and
work over the upper part until about a third or half
of the shell has been removed.
Then with a finer pair of forceps remove the sheik
membrane ; it will readily come away in strips, torn
across the long axis of the egg in a somewhat spiral
fashion. The yolk and embryo will now come into view.
It is the practice of some simply to break the egg
across and pour the yolk and white together into a
basin, very much as the housewife does. We feel sure,
however, that the extra trouble of the method we
have given will be more than repaid in the results.
During this time, and indeed during the whole
period of the examination of the embryo in situ, the
basin and its contents must be maintained, either by
renewal of the salt solution, or by the basin being
placed on a sand-bath, at about 38C.
B. Examination of the blastoderm in situ.
This may be done with the naked eye, or with a
simple lens of low power. Observe :
E. 16
242 PRACTICAL DIRECTIONS. [APP.
1. Lying across the long axis of the egg, the pellucid
area, in the middle of which the embryo may be
obscurely seen as a white streak.
2. The mottled vascular area, with the blood-vessels
just beginning to be formed.
3. The opaque area spreading over the yolk with the
changes in the yolk around its periphery.
4. (With a simple lens), the contractions of the heart;
perhaps the outlines of the head of the embryo may
be detected.
C. Removal of the embryo.
Plunge one blade of a sharp fine pair of scissors
through the blastoderm, just outside the outer margin
of the vascular area, and rapidly carry the incision
completely round. Until the circle is complete, avoid
as much as possible any agitation of the liquid in the
basin.
With a little trouble, the excised blastoderm may
now be floated into a watch-glass, care being taken to
keep it as flat as possible. With a pair of forceps or
with a needle, aided by gentle shaking, remove the
piece of vitelline membrane covering the blastoderm.
If any yolk adheres to the blastoderm, it may with
a little gentle agitation easily be washed off. Some-
times it is of advantage to suck up the yolk with a
glass syringe, replacing the fluid removed with clean
(*75 p. c.) salt solution.
The blastoderm should now be removed from the
watch-glass to a microscopic glass slide ; since it is
difficult in the former to prevent the edges of the
blastoderm from curling up.
The transference may easity be effected, if both the
watch-glass and slide are plunged into a basin of clean
warm salt solution. With a little care, the blasto-
derm can then be floated from the one to the other, and
the glass slide, having the blastoderm with its upper
surface uppermost spread flat upon it, very gently
raised out of the liquid.
APP.] SURFACE VIEW OF EMBRYO. 243
A thin ring of putty may now be placed round the
blastoderm, a small quantity of salt solution gently
poured within the ring, and the whole covered with a
glass slide, which may be pressed down until it is
sufficiently close to the embryo. The presence of any
air-bubbles must of course be avoided.
Provided care be otherwise taken to keep the
embryo well covered with liquid, the putty ring and
the coverslip may be dispensed with. They are often
inconvenient, as when the embryo has to be turned
upside down.
The object is now ready for examination with a
simple lens or with a compound microscope of low
objective. It is by far the best for the student to
begin at least with the simple lens. In order that
everything may be seen at its best, the slide should be
kept warmed to about 38, by being placed on a
hot stage.
D. S^l,rface view of the transparent embryo from
above.
The chief points to be observed, are :
1. The head-fold.
2. The indications of the amnion ; especially the false
amnion, or outer amniotic fold.
3. The neural tube : the line of coalescence of the
medullary folds, the first cerebral vesicle, the com-
mencing optic vesicles, the indications of the second
and third cerebral vesicles, the as yet open me-
dullary folds at the tail end.
4. The heart seen dimly through the neural tube ; note
its pulsation if present.
5. The fold of the somatopleure anterior to the heart
(generally very faintly shewn).
6. The fold of the splanchnopleure (more distinctly
seen) : the omphalo-mesaraic veins.
7. The protovertebrce.
162
244? PRACTICAL DIRECTIONS. [APP.
8. Indications of the omphalo-mesaraic arteries.
9. The as yet barely formed tail-fold.
10. The commencing blood-vessels in the pellucid and
vascular areas.
E. Surface view of the transparent embryo from
below.
The coverslip must now be removed and the glass
slide again immersed in a vessel of clean salt solution.
By gently seizing the extreme edge of the opaque
area with a pair of forceps, no difficulty will be found
in so floating the blastoderm, as to turn it upside
down, and thus to replace it on the slide with the
under surface uppermost.
The points which most deserve attention in this
view, are :
1. The heart : its position, its union with the omphalo-
mesaraic veins, its arterial end.
2. The fold of the splanchnopleure marking the hind
limit of the gut ; the omphalo-mesaraic veins run-
ning along its wings.
3. The protovertebrce on each side of the neural canal
behind the heart ; farther back still, the vertebral
plates not divided into protovertebrse.
F. The examination of the embryo as an opaque
object.
This should never be omitted. Many points in
the transparent embryo only become intelligible after
the examination of it as an opaque object.
Having removed the putty ring and coverslip, if
previously used, allow the blastoderm so far to become \
dry, that its edge adheres to the glass slide. Care
must of course be taken that the embryo itself does
not become at all dry. Place the glass slide with the
blastoderm extended flat on it, in a shallow vessel
containing a '1 or '5 per cent, solution of chromic acid.
.] THE EMBRYO AS AN OPAQUE OBJECT. 245
If the blastoderm be simply immersed by itself in
the chromic acid solution, the edges of the opaque
area will curl up and hide much of the embryo. The
method suggested above prevents these inconveni-
ences.
The embryo thus hardened and rendered opaque
by immersion in chromic acid, (a stay of 24 hours in
the solution will be sufficient) may be removed to a
' watch-glass, containing either some of the solution, or
plain water, and examined with a simple lens, under
a strong direct light. The compound microscope will
be found not nearly so advantageous for this purpose
as the simple lens. A piece of black paper placed under
the watch-glass, will throw up the lights and shadows
of the embryo, with benefit. The watch-glass should
have a flat bottom; or a shallow flat glass cell should
be used instead.
a. Looking at the embryo from above, observe :
1. The head-fold; the head distinctly projecting from
the plane of the blastoderm, and formed chiefly by
the forebrain and optic vesicles.
2. The elevation of the medullary canal, and the
indications of the side walls of the embryo.
3. The indications of the tail.
4. The Amnion partly covering the head. Tear it open
with needles. Observe its two folds.
b. Having turned the blastoderm upside down,
observe the following points, looking at the embryo
from below.
1. The hinder limit of the splanchnopleure in the head-
fold, marking the hind limits of the fore-gut. The
opaque folds now conceal the head almost entirely
from view.
2. The commencing tail-fold, and the shallow boat-
shaped cavity (of the alimentary canal) between it
and the head-fold. *
The student should not fail to make sketches of
246 PKACTICAL DIRECTIONS. [APP.
the embryo, both as a transparent, and as an opaque
object, seen from below as well as from above.
These sketches will be of great service to him when
he comes to study the sections of the same embryo.
G. On preparing sections of the embryo.
1. HARDENING.
a. With chromic acid.
The embryo must be immersed in the way
recommended under F, in a solution of the
strength of *1 p. c. for 24 hours. From this it
should be removed and placed in a stronger
solution (*3 p. c.) for another 24 hours. If it then
appears sufficiently hard, it may be at once
placed in alcohol of 70 p. c., in which it should
remain for one day, and then be transferred to
alcohol of 90 p. c., after remaining in which for
two days, it should lastly be placed in absolute
alcohol ; in this it can stay till required for section.
If the chromic acid has not by this time been com-
pletely got rid of, the absolute alcohol must be
changed, till the specimen is entirely free from
acid.
b. With picric add.
The best form for applying the picric acid, is
that introduced by Dr Kleinenberg.
With 100 parts of water, make a cold saturated
solution of picric acid ; add to this two parts of
concentrated sulphuric acid : filter and add to the
filtrate three times its bulk of water.
In this solution of picric acid, the embryo
must be immersed in the same manner as when
chromic acid is used (vide F). After remain-
ing in the acid for Jive hours, it is to be treated
successively with weak, strong and absolute alco-
hol, as was done in the case of the chromic acid.
Still more difficulty will probably be found in
getting rid of the picric acid, than was found to
APP.] HARDENING AND STAINING. 247
be the case with the chromic acid, and the
alcohol will probably have to be changed several
times.
c. With osmic acid.
Immerse the embryo in a '5 p. c. solution of
osmic acid ; leave it in this, covered and in the
dark, for 2J hours, and then place the embryo in
absolute alcohol, taking care completely to get
rid of the acid by washing several times with
alcohol.
The osmic acid method has the advantage of
being simpler than the others, and also of leaving
the cells in a more natural condition. Its dis-
advantages are, that it is less certain, and further,
that it is necessary to cut sections of the embryo
the next day after it has been placed in the spirit.
Otherwise it becomes too brittle ; the sections are
then not so easy to make, and not so good. It has
the further disadvantage that the outlines of the
individual cells are not so clearly brought out as
with the other two reagents.
Absolute alcohol has also been employed as a
hardening reagent, but is by no means so good
as the reagents recommended above.
2. STAINING.
In most cases it will be found of advantage to
stain the embryo. The best method of doing this,
is to stain the embryo as a whole, rather than to
stain the individual sections after they have been
cut.
Either carmine or haematoxylin may be em-
ployed. For carmine, Beales' or some other alco-
holic solution is the best. Into this the embryo
may be removed directly from the absolute alcohol,
left in the carmine for 24 hours, and then placed
again in absolute alcohol for another 24 hours
before being cut.
The best solution of hsematoxylin, one for which
PRACTICAL DIRECTIONS. [API*.
we are indebted to Dr Kleinenberg, is made in the
following way.
1. Make a saturated solution of crystallized calcium
chloride in 70 p. c. alcohol, and add alum to
saturation.
2. Make also a saturated solution of alum in 70 p. c.
alcohol, and add 1 to 2 in the proportion of 1:8.
3. To the mixture of 1 and 2 add a few drops of a
'barely alkaline saturated solution of hsematoxy-
lin.
The embryo may be removed into this directly
from the absolute alcohol, left in it for 5 hours
and placed back again in absolute alcohol for 24
hours.
Neither the carmine nor the haematoxylin will
stain if the embryo is not quite free from acid.
Only the chromic or picric acid specimens re-
quire to be stained. It is however possible to stain
the osmic acid specimens with hsematoxylin.
3. IMBEDDING.
It is hardly possible to obtain satisfactory sec-
tions of embryos, without employing the method of
imbedding now so largely employed in histological
studies.
The substances most generally used, are:
1. Paraffin, obtained by heating together five parts
of solid paraffin with one part of paraffin oil and
one part of pig's lard.
2. Wax and oil, made by heating together three parts
of common white wax and one of olive-oil.
3. Spermaceti, obtained by heating together very
thoroughly four parts of spermaceti and one of
cocoa butter, or four parts of spermaceti and one
of castor oil.
[4. Gum. The above substances are all liquid when
warm, and become solid when cold. When a
solution of gum is used as an imbedding medium,
APP.] IMBEDDING. 249
it must be hardened with, alcohol. We cannot
however recommend this substance for embryos.
For the method of employing it, see Hand-book
for ike Physiological Laboratory, p. 92.]
The exact consistency of these various mixtures
should be made to vary according to the solidity
of the embryo, by increasing or diminishing the
various constituents.
From a cold solid mass of one of the mix-
tures, cut a cubical block (about an inch cube),
and on one surface make a shallow cavity, large
enough to receive the blastoderm. Having with
a morsel of blotting-paper removed all superfluous
alcohol from the blastoderm place it quite flat
in the cavity just made, and gently pour over it
some of the mixture sufficiently heated to have
become liquid (care being taken, of course, that
it is not too hot). With a heated needle, remove
any bubbles that may make their appearance ; and
make on the block, as a guide in cutting the sections,
a mark of the exact position of the embryo. It is
sometimes of advantage to transfer the embryo from
the alcohol to some oil of cloves (when the wax and
oil is used, or to some creosote, when the paraffin is
employed), and to allow it to become saturated
with that substance, before placing it in the block.
The adhesion of the imbedding material to the
object imbedded, is thus rendered more complete.
This method serves fairly well, when there are
no cavities in the object into which it is necessary
for the mixture to penetrate. When these exist,
the following method (again Dr Kleinenberg's) will
be found the most satisfactory.
Remove the object from the absolute alcohol
and place it directly into Bergamot oil, and leave it
there, till the oil has completely penetrated through
it. In the mean time, prepare a small paper box,
by bending up the sides and folding in the corners
of a piece of stiff paper, and pour on the bottom of
it a thin coating of the mixture of spermaceti and
castor oil. When this has become solid, place the
250 PRACTICAL DIRECTIONS. [APP,
embryo flat upon it, having removed as much super-
fluous Bergamot oil as possible, and then pour in
some more of the spermaceti mixture, which should
be as hot as can be used without injuring the object.
With a hot needle, move the object about so as to
get rid of all air bubbles, and to assist the sperma-
ceti in penetrating all parts of the embryo. Finally,
before the spermaceti becomes solid, place the
embryo in the position required for subsequently
making sections ; and when the whole becomes
solid, make a mark over the position of the embryo.
It is better to soak the object in the hot spermaceti
before finally imbedding it, but the manipulation of
this is rather difficult with embryos of this age.
If successfully imbedded, the spermaceti will be
found to have penetrated through and through the
embryo ; and the method has the great merit of per-
mitting the imbedding medium to be quite easily
dissolved away from the sections, by a few minutes'
immersion in a mixture of four parts of turpentine
to one of creosote. With the other imbedding
materials of which we have spoken, this cleaning
of the sections is very troublesome and difficult, and
liable to result in injury to the specimens.
4. CUTTING SECTIONS.
When the imbedding block is cold pare away
the edges and then gradually slice it away until the
edge of the area opaca appears. The sections must
then be made more carefully, and each one ex-
amined till the actual body of the embryo is arrived
at.
It is best to begin with transverse vertical
sections which may commence either at the tail or
head end. The latter is preferable, since the bit
which is last cut is apt to slip.
A section instrument may be used ; but we our-
selves very much prefer a simple razor provided
with heavy fixed handle (i.e. with no hinge) held
in the hand.
APR] CUTTING SECTIONS. 251
In any case the cutting-blade should for the
first method of imbedding be kept freely wetted
with spirit; and each section, as it is made, carefully
floated on to a glass slide and then, the spirit
having been removed, mounted in glycerine, or
treated with creosote or oil of cloves and turpentine
and mounted in balsam or dammar. It is well to
guard the section with a small diaphragm of paper
placed under the cover-slip.
When the object has been imbedded in sper-
maceti in the way recommended above, the block
of spermaceti and the razor should be moistened
with olive oil and not with spirit. The sections
must be floated from the razor on to the glass
slip in the ordinary way and then treated with a
mixture of turpentine (4 parts) and creosote (1
part) till all the oil and the spermaceti are removed.
They may then be mounted in Canada balsam or
dammar in the usual way. Even when the other
methods of imbedding spoken of above are adopted,
it will frequently be found advantageous to moisten
the razor with creosote or oil of cloves rather than
with spirit.
Whichever method be followed, a series of sec-
tions, each as thin as possible, should be obtained
and carefully numbered, from head to tail or vice
versa. They should at first at least all be retained
for study, and not even the fragmentary ones
thrown away, these being often the most instructive.
The following transverse sections will perhaps
be the most instructive.
1. Through the optic vesicles, shewing the optic stalks.
2. Through the hind-brain, shewing the auditory sacs.
3. Through the middle of the heart, shewing its rela-
tions to the splanchnopleure and alimentary canal.
4. Through the point of divergence of the splanchno-
pleure folds, shewing the venous roots of the heart.
5. Through the dorsal region, shewing the medullary
252 PRACTICAL DIRECTIONS. [APP.
canal, proto vertebrae and commencing cleavage of
the mesoblast.
6. Through a point where the medullary canal is still
open, shewing the mode in which its closing takes
place.
A good series of transverse sections having been
obtained, longitudinal vertical ones may be made
in a similar manner. These are however much
more difficult to manage ; and are moreover chiefly
of use to compare with the transverse ones.
GG. Preservation of the embryo as a whole.
Embryos of this or an earlier day may be easily
preserved whole as microscopic objects; but the value
of such preparations is very slight, and they are per-
haps not worth the trouble. The best method is
probably to place the embryos in the picric acid
solution for a short time, and then successively in
weak, strong and absolute alcohol and finally to mount
them, in glycerine or, after staining and treatment
with oil of cloves, in balsam. For shewing some
points, treatment with osmic acid for a short time and
then with alcohol and finally with glycerine may be
adopted.
Whole embryos of a later date cannot be satis-
factorily preserved as microscopic objects.
III. Examination of an Embryo of about 48 50 hours.
A. Opening the egg as in II. A.
B. Examination of the blastoderm in situ.
Observe
1. The form of the embryo, which is much more
distinct than at the earlier stage.
2. The beating of the heart.
3. The general features of the circulation.
APP.] EMBRYO OF THE THIRD DAY. 253
C. Removal of the embryo from the yolk, as in
II. C.
D. Surface view of the transparent embryo from
above.
Notice:
1. General form of the embryo.
a. Commencing cranial flexure.
b. The tail and side folds.
2. Amnion. Notice the inner and outer (false amnion)
limbs and remove them with a needle. When the
amnion has been removed the features of the
embryo will be much more clearly visible.
3. The organs of sense.
a. Eye. Formation of the lens already nearly
completed.
b. Auditory involution, now a deep sac with a
narrow opening to the exterior.
4. The brain.
a. The vesicles of the fore-, mid~, and Amc?-brain.
b. The two cerebral lobes, which have been budded
off from the fore-brain.
c. The cranial flexure taking place at the mid-
brain.
E. Transparent embryo from below.
Manipulation as in II. E.
Notice :
1. The increase of the head-folds of the somatopletire
and splanchnopleure, especially the latter, and the
commencement of these folds at the tail.
254 , PRACTICAL DIRECTIONS. [APP.
2. The now GC-shaped heart; for further particulars
vide Chap. iv. 18.
3. The commencing 1st and 2nd visceral clefts and
the aortic arches.
4. The circulation of the yolk sac, vide Fig. 23. Make
out all the points there shewn and ascertain
by examination that what have been called the
veins and arteries in that figure, are truly such.
F. The embryo as an opaque object.
Treatment as in II. F.
FROM ABOVE :
Observe the amnion, which is a very conspicuous
object, and remove it with needles if not done pre-
viously. The external form of the brain and the
auditory sac appear very distinctly.
FROM BELOW:
Observe the nature of the head- and tail-folds,
which are much more easily understood from the
opaque than from the transparent embryos.
Observe also the alimentary canal, the widely open
hind end of the fore-gut, and the front end of the as
yet very short hind-gut.
G. Sections.
Manipulation as in II. G. The embryo can still
be stained as a whole.
The more important sections to be preserved, are
1. Through optic lobes, shewing :
a. The formation of the lens.
b. The involution of the primary optic vesicle.
c. The constriction, especially from above, of the
optic stalk.
APR] EMBRYO OF THE THIRD DAY. 255
2. Through auditory sac, shewing:
a. Auditory sac still open.
b. The thin roof and thick sides of the hind-brain.
c. Notochord.
d. Heart.
e. Closed alimentary canal.
3. Through dorsal region, shewing the general appear-
ance of a section of an embryo at this stage, which
should be compared with a similar section of the
earlier stage.
It shews:
a. The commencement of the side folds ; the
alimentary canal still however open below.
b. The Wolffian duct lying close under the epiblast
on the outside of the pro to vertebrae.
c. The notochord with the aortae on each side.
IV. Examination of an Embryo at the end of the third
day.
A. Opening the egg, as in II. A.
B. Examination of the blastoderm in situ.
Observe:
1. The great increase of the vascular area both in size
and distinctness. The circulation is now better
seen in situ than after the blastoderm has been
removed.
2. That the embryo now lies completely on its left
side and that it is only connected with the yolk-sac
by a somewhat broad stalk.
C. Removal of the embryo. See II. C.
It is now unnecessary to remove the whole of the
blastoderm with the embryo; indeed it is better to
256 PRACTICAL DIRECTIONS. [APP.
cut away the vascular area unless it is wanted for
examination.
D. S^t,rface view of the transparent embryo.
Since the embryo now lies on its side we shall
not have to speak of the view from above and below.
The views from the two sides differ chiefly as to the
appearance of the heart.
The embryo {freed from the blastoderm and the
amnion) is to be floated on to a glass slide in the
usual way. It is necessary to protect it while under
examination, with a cover-slip, which must not be
allowed to compress it. To avoid this, we have found
it a good plan to support the cover-slip at one end
only, since by moving it about when thus supported,
a greater or less amount of pressure can be applied
at will to the object.
The details which can at this stage be seen in a
transparent embryo are very numerous arid we re-
commend the student to try and verify everything
shewn in Fig. 24. Amongst the more important and
obvious points to be noticed are
1. The increase of the cranial flexure and the lody-
flexure.
2. The condition of the brain. The mid-brain now
forms the most anterior point of the head.
The fore-brain consists of the inconspicuous
vesicle of the third ventricle and the two large
cerebral lobes.
The hind-brain consists of a front portion, the
cerebellum with a thickened roof; and a hinder
portion, the fourth ventricle with a very thin and
delicate roof.
3. Organs of sense.
The eye especially is now in a very good state
to observe. The student may refer to Fig. 31,
and the description there given. .
rP.] EMBRYO AT THE END OF THE THIRD DAY. 257
The ear-vesicle will be seen either just closing
or completely closed.
4. In the region of the heart attention must also be
paid to:
a. The visceral clefts.
b. The investing-mass, i.e. the growth of mesoblast
taking place around the end of the notochord,
c. The condition of the heart.
5. In the region of the body the chief points to be
observed are:
a. The increase in the number of the protovertebrcv.
b. The Wolffian duct, which can be seen as a streak
along the outer side of the hinder protovertebra3.
c. The allantois, which is now a small vesicle lying
between the folds of the somatopleure -and
splanchnopleure at the hind end of the body, but
as yet hardly projects beyond the body cavity.
F. The embryo as an opaque object.
Preparation as in II. F.
The general form of the embryo can be very satis-
factorily seen when it is hardened and examined as
an opaque object ; but the most important points to be
made out at this stage in the hardened specimens are
those connected with the visceral clefts and folds and
the mouth.
If the amnion has not been removed it will be
necessary to pick it completely away with needles.
Without further preparation a view of the visceral
folds and clefts may be obtained from the side; but a
far more instructive view is that from below, in order
to gain which the following method may be adopted.
Pour a small quantity of melted black wax (made
by mixing together lampblack and melted wax) into
a watch-glass, using just enough to cover the bottom
of the glass. While still soft make a small depression
in the wax with the rounded end of a pen-holder or
handle of a paint-brush and allow the wax to cool.
v 17
258 PRACTICAL DIRECTIONS. [APP.
In the meantime cut off the head of the hardened
embryo by a sharp clean transverse incision carried
just behind the visceral clefts, transfer it to the watch-
glass and cover it with water or spirit. By a little
manipulation the head of the embryo may now be
shifted into the small depression in the wax, and thus
be made to assume any required position. It should
then be examined with a simple lens under a strong
reflected light, and a drawing made of it.
When the head is placed in the proper position,
the following points may easily be seen.
1. The opening of the mouth bounded below by the
first pair of visceral folds, and commencing to be
enclosed above by the now very small buds which
are the rudiments of the superior maxillary pro-
cesses. Compare Fig. 38.
2. The second and third visceral arches and clefts.
3. The nasal pits.
G. Sections. Manipulation as in II. G.
The embryo can still be stained as a whole.
The most important sections are :
1. Through the eyes in the three planes, vide Fig.
30, D. E. F.
2. Through the auditory sac.
3. Through the dorsal region, shewing the general
changes which have taken place.
Amongst these, notice
a. The changes of the protovertebrce : the commencing
formation of the muscle-plates.
b. The position of the Wolffian duct and the forma-
tion of the germinal epithelium.
c. The aortai and the cardinal veins.
d. The great increase in depth and relative diminu-
tion in breadth of the section.
APP.] EMBRYO OF THE FOURTH DAY. 259
V. Examination of an Embryo of the Fourth Day,
A. Opening the egg, as in II. A.
Great care will be required not to injure the blas-
toderm, which now lies close to the shell-membrane.
B. Examination in situ. Observe :
1. The now conspicuous amnion.
2. The allantois, a small, and as yet hardly vascular
vesicle, beginning to project from the embryo into
the space between the true and the false amnion.
3. The rapidly narrowing somatic stalk.
C. Removal of the embryo, as in II. C. and IV. C.
The remarks made in the latter place apply with
still greater force to an embryo of the fourth and
succeeding days.
D. S^crface view of the transparent embryo. For
manipulation, vide IV. D.
The points to be observed are :
1. The formation of the fifth, seventh, and ninth cranial
nerves.
To observe these, a small amount of pressure is
advantageous.
2. The formation of the fourth viscefal cleft, and the
increase in size of the superior maxillary process.
3. The formation of the nasal pits and grooves.
4. The great relative growth of the cerebral lobes and
the formation of the pineal gland from the roof
of the vesicle of the third ventricle.
5. The great increase in the investing mass.
6. The formation and growth of the muscle-plates,
which can now be easily seen from the exterior.
7. The allantois. Make out its position and mode of
opening into the alimentary canal.
260 PRACTICAL DIRECTIONS. [APP
F. The embryo as an opaqiie object. Manipulation
as II. F. For mode of examination vide
IV. F.
The view of the mouth from underneath, shewing
the nasal pit and grooves, the superior and inferior
maxillary processes and the other visceral folds and
clefts, is very instructive at this stage. Compare
Fig. 48.
G. Sections. Manipulation as in II. G.
A slightly stronger solution of chromic acid may
be used than for the younger embryos.
The student will most probably find that he can
still stain the embryo as a whole, especially if he
employs hsernatoxylin. If this cannot be done, the
sections must be stained individually after being cut.
The most important sections are,
1. Through the eyes.
2. Transverse section immediately behind the visceral
arches, shewing the origin of the lungs.
3. Transverse section just in front of the umbilical
stalk, shewing the origin of the liver.
4. Transverse section at about the centre of the dorsal
region, to shew the general features of the fourth
day. Compare Fig. 47.
Amongst the points to be noticed in this section, are
a. Muscle-plates.
b. Spinal nerves and ganglia.
c. "Wolffian duct and bodies.
d. Miiller's duct.
e. Mesentery.
f. Commencing changes in the spinal cord.
5. Section passing through the opening of the allantois
into the alimentary canal.
For the points to be observed in embryos of the
fifth and sixth days, the student must consult the
chanters devoted to those davs.
IPP.] BLASTODERM OF TWENTY HO UBS. 261
In the hardened specimens, especial attention
should be paid to the changes which take place in
the parts forming the boundaries of the mouth.
In making sections, it will probably be found
impossible to stain the embryo as a whole; in
this case, the individual sections must be stained
separately.
VI. Examination of a Blastoderm of 20 hours.
A. Opening the egg, as in II. A.
B. Examination in situ.
It will not be found possible to make out anything
very satisfactory from the examination of a blasto-
derm in situ at this age. The student will however
not fail to notice the halones, which can be seen
forming concentric rings round the blastoderm.
C. Removal of the embryo.
Two methods of hardening can be adopted at
this age. One of these involves the removal of the
blastoderm from the yolk, as in II. C. In the other
case, the yolk is hardened as a whole. The latter
of these is the most satisfactory method for sections ;
but if employed, the embryo cannot be viewed as a
transparent object.
In the cases where the blastoderm is removed
from the yolk, the manipulation is similar to that
described under II. C, with the exception of more
care being required in freeing the blastoderm from
the vitelline membrane.
D. Surface view transparent, from above.
Observe :
1. The medullary groove between the two medullary
folds, whose hind ends diverge to enclose between
them the end of the primitive groove.
2. The head-fold at the end of the medullary groove.
3. The one or two pairs of protovertebrce flanking
the medullary groove.
262 PRACTICAL DIRECTIONS. [APR
4. The notochord as an opaque streak along the floor
of the medullary groove.
E. Surface view transparent, from below.
Same points to be seen as from above, but less
clearly.
F. Embryo as an opaque object.
As an opaque object, whether the embryo is hard-
ened in situ or after being removed from the yolk,
the same points are to be seen as when it is viewed
as a transparent object, with the exception of the
notochord and protovertebrse (vide D). The various
grooves and folds are however seen with far greater
clearness.
G. Sections.
Two methods of hardening may be employed;
(1) with the embryo in situ, (2) after it has been
removed.
To harden the blastoderm in situ the yolk must
be hardened as a whole. After opening the egg either
leave the yolk in the egg-shell or pour it out into a
Berlin capsule; in any case freeing it as much as
possible from the white, and taking especial care to
remove the more adherent layer of white which im-
mediately surrounds the yolk.
Place it in a weak solution of chromic acid (first
of 1 p.c. and then of *5 p.c.) with the blastoderm upper-
most and leave it in that position for two or three days.
Care must be taken that the yolk does not roll
about ; the blastoderm must not be allowed to alter
its position: otherwise it may be hard to find it when
everything has become opaque. If at the end of the
second day the blastoderm is not sufficiently hard,
the strength of the solution should be increased and
the specimen left in it for another day.
After it has become hardened by the chromic acid,
the yolk should be washed with water and treated
successively with weak and strong spirit, vide II. G.
After it has been in the strong spirit (90 p. c.) for two
days, the vitelline membrane may be safely peeled off
APP.] HAKDENING THE WHOLE YOLK. 263
and the blastoderm and embryo will be found in
situ. The portion of the yolk containing them must
then be sliced off with a sharp razor, and placed in
absolute alcohol.
The staining, &c. may be effected in the ordinary
way.
If osmic acid, which we believe will be found
especially serviceable for these early stages, is em-
ployed, it will be necessary to remove the blastoderm
from the yolk before treating it with the reagent.
The following transverse sections are the most im-
portant at this stage :
1. Through the medullary groove, shewing
a. The medullary folds with the thickened meso-
blast.
b. The notochord under the medullary groove.
c. The commencing cleavage of the mesoblast.
2. A section through the region where the medullary
folds diverge, to enclose the end of the primitive
groove, shewing the greatly increased width of the
medullary groove, but otherwise no real alteration
in the arrangement of the parts.
3. A section through the front end of the primitive
groove with the so-called axis cord underneath it,
while on each side of it are still to be seen the
medullary folds.
4. A section through the primitive groove behind this
point, shewing the typical characters of the primitive
groove.
VII. Examination of an unincubated Blastoderm.
A. Opening the egg. Vide II. A.
B. Examination of the blastoderm in situ.
Observe the central white spot and the peripheral
more transparent portion of the blastoderm and the
halones around it.
264< PRACTICAL DIRECTIONS. [APP.
C. Removal of the blastoderm. Vide VI. C.
With the unincubated blastoderm still greater care
is required in removal than with the 20 hours' blasto-
derm, and there is no special advantage in doing so
unless it is intended to harden it with osmic acid.
D. Surface view transparent from above.
Observe the absence of the central opacity.
E. Sitrface view transparent from underneath.
Nothing further to be observed than from above.
F. As an opaque object.
There is nothing to be learnt from this.
G. Sections.
Manipulation as in VI. G.
Only one section is required, viz. one taken through
the centre of the blastoderm, shewing:
a. The distinct epiblast.
b. The lower layer cells not as yet differentiated
into mesoblast and hypoblast.
c. The thickened edge of the blastoderm.
d. The segmentation cavity and formative cells.
VIII. Examination of the process of Segmentation.
To observe the process of segmentation it will be
found necessary to kill a number of hens which arc
laying regularly. The best hens lay once every 24<
hours, and by observing the time they usually lay (and
they generally lay pretty regularly about the same
time), a fair guess may be made beforehand as to
the time the egg has been in the oviduct. By this
means a series of eggs at the various stages of seg-
mentation, may usually be obtained without a great
unnecessary sacrifice of hens. For making sections,
the yolk must in all cases be hardened as a whole,
which may be done as recommended in VI. G.
APP.] SEGMENTATION. 2G5
Chromic acid is an excellent reagent for this and it
will be found very easy to make good sections.
In the sections especial attention should be paid,
1. To the first appearance of nuclei in the segments,
and their character.
2. To the appearance of the horizontal furrows.
3. As to whether new segments continue to be formed
outside the limits of the germinal disc, or whether
the fresh segmentation merely concerns the already
formed segments.
4. In the later stages, to the smaller central and larger
peripheral segments, both containing nuclei.
For surface views, the germinal disc, either fresh
or after it has been* hardened, can be used. In
both cases it should be examined by a strong
reflected light. The chief point to be noticed is the
more rapid segmentation of the central than of the
peripheral spheres.
IX. Examination of the later changes of the Embryo.
For the later stages, and especially for the develop-
ment of the skull and the vascular system of the body
of the chick, it will be found necessary to dissect the
embryo. This can be done either with the fresh
embryo or more advantageously with embryos which
have been preserved in spirit.
If the embryos are placed while still living into
spirit a natural injection may be obtained. And such
an injection is the best for following out the arrange-
ment of the blood-vessels.
Sections of course will be available for study,
especially when combined with dissections.
X. Study of the development of the Blood-vessels.
Observations on this subject must be made with
blastoderms of between 30 40 hours. These are to
be removed from the egg, in the usual way (vide II. A.
and C.) spread out over a glass slip and examined
from below, vide II. E.
266 PRACTICAL DIRECTIONS. [APP.
The blastoderm when under examination must be
protected by a cover-slip with the usual precautions
against pressure and evaporation, and a hot stage
must also be employed.
Fresh objects so prepared require to be examined
with a considerable magnifying power (400 to 800
diameters). From a series of specimens between 30
and 40 hours old all the points we have mentioned in
Chapter IV. 6 can without much difficulty be observed.,
Especial attention should be paid in the earlier
specimens to the masses of nuclei enveloped in proto-
plasm and connected with each other by protoplasmic
processes ; and in the later stages to the conversion of
these nuclei into blood corpuscles and of the proto-
plasmic processes into capillaries, with cellular walls.
Blastoderms treated in the following ways may be
used to corroborate the observations made on the
fresh ones.
1. With gold chloride.
Immerse the blastoderm in gold chloride (*5 p. c.)
for one minute and then wash with distilled water
and mount in glycerine and examine.
By this method of preparation, the nuclei and
protoplasmic processes are rendered more distinct,
without the whole being rendered too opaque for
observation.
The blastoderm after the application of the gold
chloride should become a pale straw colour ; if it
becomes in the least purple, the reagent has been
applied for too long a time.
2. With potassium bichromate.
Immerse in a 1 p. c. .solution for one day and then
mount in glycerine.
3. With osmic acid.
Immerse in a "5 p. c. solution for half an hour and
then in absolute alcohol for a day, and finally
mount in glycerine.
INDEX.
A.
AFANASSIEFP, 64, 71, 72.
Air-chamber, 12.
Albumen, 13.
Aliethmoid cartilage, 236.
Alimentary canal, 61.
Alinasal cartilages, 227, 231, 236.
Aliseptal cartilages, 231, 236.
Alisphenoid, 232.
Allan toic arteries, 168.
Allantoic veins, 210.
Allantois, 41, 147150, 174, 201,
202, 203.
Amnion, 3941, 57> 8 3 H 1 * 2Or ,
202 ; cavity of, 40.
Amniotic fluid, 41 ; sac, 41.
Annuli fibrosi, 158.
Anterior fissure, 186, 187.
Anus, 183.
Aorta, dorsal, 121 ; primitive aortse,
66, 79.
Aortic arches, Si, 121, 213, 217, 218.
Aquapendente, FABRICIUS of, 2.
Aqueductus vestibuli, in, 115.
Area opaca, 16, 18; pellucida, 16.
AEISTOTLE, 2.
ArteriaB collaterals colli, 217.
Arterial arches, 168, 169; arterial
system, 213 220.
Arteries; allantoic 168; iliac 168,
214 ; omphalo-mesaraic 66, 80, 168,
202, 214; subclavian 217; umbilical
1 68 ; vertebral 217.
Auricles, 63, 79, 173.
Axis-cord, 48.
B.
BABUCHIN, 106.
BAER. See VON BAER.
Basibranchial, 230.
Basi-hyal, 230.
Basioccipital, 236.
Basipterygoid, 236.
Basisphenoid, 231.
Basisphenoidal ossicles, 235.
Basitemporals, 232, 235, '236.
Beak, 205.
Blastoderm, 5, 14, 16 18, 174, 241,
242, 252 ; of 20 hours, 261 ; uniii-
cubated 263.
Blood-vessels, 66 72 ; study of de-
velopment of, 265, 266.
Body-folds, 31.
BOETTCHER, 113, 114, Il6.
Bone; metatarsal 176; occipital 227 ;
tarsal, 176.
BONNET, 4.
BORNHAUPT, 149, 162.
Branchial clefts, 119.
Branchial fold, 119.
Bulbus arteriosus, 63, 79, 123, 173,
190194, 214, 215.
C.
Canal; alimentary 61; semicircular,
115.
Canales Botalli, 219, 220.
Canalis auricularis, 1 23, 1 72 ; reuniens,
in.
Capsules, periotic, 227.
Cardinal veins, 124.
Carotids, 169, 213.
Carpals, 175, 176.
Carpo-metacarpus, 175.
Carpus, 175.
Cartilage, 230 232 ; aliethmoid 236 ;
alinasal227, 231, 236; aliseptal 231,
236; ethmoido-nasal 236; ethmo-
presphenoid 235, 236; Meckel's,
179, 182, 229, 230; paired rods of,
227; pre-nasal, 231, 232.
268
INDEX.
Cartilage-bone, 178.
Cartilaginous rods, 228; skeleton,
206 ; skull, 230 233.
Cavity of the amnion, 40.
Cell-mass, intermediate, 82, 136, 161,
165.
Centrale, 176.
Cerato-branchial, 230.
Cerato-hyals, 230.
Cerebellum, 93.
Cerebral vesicles, 58 60.
Chalazee, 13.
Change of position of embryo, 87.
Chorion, 32, 42, 203.
Choroid, 102.
Choroidal fissure, 99 101.
Cicatricula, 14. See Blasto'derm.
Circulation, 79, 81, 168173, 220
223.
CLARKE, Lockhart, 153, 185, 187;
L. C., 186.
Cleavage of the mesoblast, 38, 57, 65,
202, 203.
Clefts; branchial, 119; visceral, 119,
204.
Clinoid walls 231.
Cochlea, in, 112.
Condyle, occipital, 230, 233, 236.
Cones, 106.
Coni vasculosi, 167.
Corpora bigemina, 93.
CORTI, rods of, 117.
COSTE, 22, i8r.
Cranial flexure, 78, 87, 89, 143;
cranial nerves, 137, 138, 147.
Cranium, 177 179, 225 238.
Crura cerebii, 93.
Curvature of the body, 87,
Cutting sections, 250 252.
D.
DARESTE, 15.
DESCEMET, membrane of, 102.
Descent of the ovum, 21.
Digits, 175.
Disc, germinal, 19.
Discus proligerus, 19.
DOBRYNIN, 125, 148, 149.
DoLLINGER, 5.
Dorking fowls, 176.
Dorsal aorta, 121.
Duct of Miiller, 162, 163, 168; um-
bilical duct, 143, 174. See Wolffian
duct.
Ductus arteriosi, 219; Botalli, 219;
cochlearis, in, 115; Cuvieri, 124,
I7i;venosus, 123, 170, 210.
Duodenum, 128.
DURSY, 50, 78, 82, 91, 156.
E.
Ear, 78, in; 117, 237.
Ectosteai ossification, 233, 237.
Ectostosis, 233.
Egg-shell, n.
Elbow, 175.
Embryo as an opaque object, 245,
2 54> 2 57> 2 6o, 262; of the third
day, examination of, 252 258; of
the fourth day, 259 261.
Embryology, meaning of, i.
Embryonic sac, 35; shield, 44.
Endostosis, 233.
Epiblast, 27, 44, 56, 197.
Epididymis, 167.
Epigenesis, 3.
Epiotic, 238.
Episkeletal muscles, 159.
Epithelium, germinal, 160, 165.
Ethmoid, 237.
Ethmoido -nasal cartilage, 236.
Ethmo-presphenoid cartilage, 235,
236; plate, 231.
Eustachian valve, 195.
Evolution, 3.
Examination of the blastoderm in
situ, 241, 242, 252, 255.
Exoccipital plates, 236.
Exoccipitals, 230.
Eye, 117.
Eyeball, 94.
F.
FABRICIUS, 2, 3.
Feathers, 205.
Fenestra ovale, 232; rotund um, 232.
Fenestrae, 236.
Fibulare, 176.
Fissure; anterior, 186, 187 ; choroidal,
99 101 ; posterior, 187.
Foetal appendages, 201.
Foot, 175, 176.
Foramen ovale, 194; occipital, 227.
Fore-brain, 78, 91.
Foregut, 62,
Formative cells, 17, 26, 44.
INDEX.
2C9
Fourth ventricle, 94.
Fretum Halleri, 172.
F rentals, 235.
Fronto-nasal process, 120, 145.
G.
Gall-bladder, 133.
(^asserian ganglion, 147.
GEGENBAUR, 154, 157, 158, 228, 229.
Genital ridge, 164.
Germinal disc, 19 ; epithelium, 16.0,
165; spot, 20; vesicle, 20.
Gland, pineal, 91.
Glossopharyngeal nerve, 147.
GOETTE, 25, {,0, 72, 93, 128, 129, 131,
J 33, H7.
Graffian follicle, 166, 167.
Grey column, 184, 185 ; grey matter,
184, 185, 186, 187, 189.
H.
Hcematoxylin, solution of. 247, 248.
Halones, 52.
HALLER, 4, 5.
Hand, 175, 176.
Hardening, 246, 247.
HARVEY, 2, 3.
HASSE, 112, 155.
Head, 145.
Head-fold, 29, 33, 34, 4 8, 57.
Heart, 6366, 122, 123, 172, 173,
190 196.
Hen's egg, structure of, TI 19;
changes in, before it is laid, 19 26.
HEN SEN, 82.
Hepatic veins, 210.
HEEING, 133.
Hind -brain, 78, 93.
His, 16, 48, 52, 55, 64, 68, 71, 72,
82, 83, 124, 134, 149, 153, 156,
159, 168.
HOPPE-SEYLER, 15.
HUXLEY, 76, 90, 137, 159, 179, 181,
182, 228.
Hypoblast, 27, 45,^51, 56, 198.
Hypophysis cerebri, 91.
I.
Iliac arteries, 168, 214.
Imbedding, 248 250.
Incubation, 2742.
Incubators, 239, 240.
Infundibulum, 91, 92.
Intermediate cell-mass, 82, 136, 161,
165.
Intermedium, 175, 176.
Investing mass, 177, 178, 225 227.
Iter a tertio ad quartum ventriculum,
93-
J.
JAEGER, 158.
Jugal, 235.
Jugular vein, 207.
K.
KAPFF, 167.
Kidneys, permanent, 163, 164.
KLEIN, 65, 72.
KLEINENBERG, 246, 248, 249.
Knee, 175.
KOELLIKER, 6, 71, 82, xor, ioS, 133,
134, 159, 181.
KUPITER, 163.
L.
Lacrymals, 237.
Laminae dorsalts, 48.
Lateral column, 186, 187.
Lateral folds, 76; plate, 54; lateral
ventricle, 91.
Lens, 98, 107, 108.
Lens-capsule, ioS, 109.
LEOPOLD, 167.
LlEBERKUEHN, 98, IOI, IO2, IO7, ioS.
Ligamentum suspansoriurn, 158.
Limbs, 143, 145, 174, 175.
Liquor amnii, 41.
Liver, 131 133.
Lungs, 129 131.
Lung- vesicle, primary, 131.
M.
MALPIGHI, 3.
Malpighian bodies, 164.
Mandibular arch, 230.
Manus, 175, 176.
Marginal process, 233.
Maxillae, 120.
Maxillares, 233.
Maxillary processes, 179,229.
Maxillo-palatine, 235.
Meatus venosus, 123, 169, 209.
270
INDEX.
MECKEL'S cartilage, 179, 182, 229,
230.
Medulla oblongata, 93.
Medullary canal, 53; folds, 37, 48,
57; groove, 48, 57.
Membrana limitans externa, 106.
Membrane of Descemet, 102 ; of
Reissner, 112; vitelline membrane
13, 20.
Membrane-bones, 178, 233, 237, 238.
Membranous labyrinth, 1 1 1 113.
Meniscus, 158.
Mesenteric veins, 171, 210.
Mesentery, 127.
Mesoblast, 27, 45, 56, 196, 198, 199.
Metacarpals, 175, 176.
Metatarsal bones, 176.
Metatarsus, 176.
Mid-brain, 78, 93, 145*
MIESCHER, 15.
Mouth, 179, .182, 183.
Movements of the embryo, 223, 224.
Mucous layer, 45.
MUELLER, 92, 93, 133, 134, 156, 157,
228; duct of M. 144; M.'s duct,
126, 162 ; fibres of M., 107.
Muscles, episkeletal, 159.
Muscle-plates, 135, 136, 159.
N.
Nails, 205.
Nasal labyrinth, 181 ; pits, 117, 145;
processes, 180, 233.
Nasals, 235.
NATHUSIUS, ir.
Neck, 119.
Nerves; cranial, 137, 138, 147;
glossopharyngeal, 147; optic, 107;
pneumogastric, 147.
Neural canal, 53; tube, 37.
Notochord, 49, 57, 78, 156, 157.
Nucleus of Pander, 15; nucleus pul-
posus, 158.
O.
Occipital bone, 227.
Occipital condyle, 230, 233, 236.
Occipital foramen, 227.
OELLACHER, 20, 21, 22, 25.
(Esophagus, 127.
Olfactory vesicle, 117.
Omphalo-mesaraic arteries, 66, 80,
168, 202, 214; veins, 63, 69, 80,
170, 202.
Opening the egg, 240, 241.
Opisthotic, 237.
Optic cup, 97, 103; nerve, 107;
vesicles, 76, 77, 94, 95, 96.
Osseous labyrinth, in 114.
Ossification, 206; of the cranium, 233,
235 ; of the ear, 237.
Otic vesicle, in.
Ovarian ovum, 19.
Oviduct, 20, 1 68.
Ovum, descent of the, 21 ; primordial
ova, 1 66, 167.
P.
Paired rods of cartilage, 227.
Palatine, 233.
Palatine rod, 230, 232.
Pancreas, 133.
PANDER, 5,71; nucleus of P., 15, 17.
Parasphenoid, 235.
Parietals, 235.
PARKER, 177, 179, 225, 226, 228, 229,
231, 232, 234, 236.
Parostosis, 233.
Parovarium, 167.
Pecten, 105.
Pellucid area, 57. See Area pellucida.
PEREMESCHKO, 133.
Pericardium, 194.
Periotic capsules, 227.
Pes, 176.
PFLUEGER, 167.
Phalaijges, 176.
Pineal gland, 91.
Pituitary body, 91, 92, 227; di-
verticulum, 92, 93; space, 178,
227, 232.
Pleuroperitoneal cavity, 38, 54.
Pneumogastric nerve, 147.
Portal vein, 210.
Posterior fissure, 187.
Post-frontal, 237.
Premaxillaries, 233, 237.
Prenasal cartilage, 231, 232.
Preservation of the embryo as a whole,
252.
Primary lung-vesicle, 131.
Primitive aortse, 66, 79; groove, 47,
57; streak, 46, 47, 50.
Primordial ova, 166, 167.
Prootic, 237.
Proto vertebrae, 55, 57, 134, 151
159-
Pterotic, 237.
INDEX.
271
Pterygoid rod, 230, 232.
Pterygo- palatine rod, 179.
Punctum saiieus, 2.
PURKINJE, 6.
Q.
Quadrate, 179, 230, 232, 236.
Quadrato-j ugals, 235.
Radiale, 175.
RATHKE, 6, 91, 92, 177, 215, 216,
217, 225, 228.
Recessus vestibuli, m, 115.
REICHERT, 91, 148.
REISSNER, membrane of, 112.
REMAK, 6, 64, 71, 72, 78, 82, 97, ror,
1 06, 1 08, 133, 134, 138, 147, 148,
150, 153, 163.
Removal of the embryo, 242, 261.
Retina, 105.
Ribs, 158.
Rods and cones, 106; rods of Corti,
117 ; paired rods of cartilage, 227.
ROMITI, 83, 167.
ROSENBERG, 175, 176.
Rostrum, 232, 235, 236.
S.
Sac; amniotic 41 ; embryonic 35.
Sacculus hemisphericus, 115.
SCHENK, 133, 137.
SCHULTZE, Max, 1 06.
SCHWARCK, 155, 157.
Sclerotic, 102.
Sections, 250252, 254, 255, 258, 260,
261, 262, 263, 264.
Segmentation, 22 26, 264, 265 ; se-
condary of vertebral column, 155.
Segmentation-cavity, 25.
Semicircular canals, 115.
Semilunar valves, 191, 19?.
Seminiferous tubules. See Tubuli
seminiferi.
Septum nasi, 237 ; septum of the
bulbus arteriosus, 215; ventricular
septum 172, 173.
SERNOFF, 162, 167.
Serous cavity, 38 ; layer, 45.
Sexual eminence, 166.
Shell- membrane, 12.
Sinus rhomboidalis, 60, 190; termi-
nalis, 69; venosus, 123, 170.
Skull, 177179, 225238.
SMIDT, 91.
Somatic stalk, 39, 143.
Somatopleure, 38, 54, 57.
Spinal cord, 183 190; ganglia, 152,
153.
Splanchnic stalk, 39, 141, 174.
Splanchnopleure, 38, 54, 57.
Spleen, 133.
Splint-bones, 233, 238.
Splitting of the mesoblast. See Cleav-
age.
Squamosal, 235.
Staining, 247.
Stapes, 232.
Stomach, 128.
STRICKER, 133.
Stroina, 166.
Subclavian arteries, 217.
Summary of changes during the first
day, 56, 57; second day, 83; third
day, 139, 140; fourth day, 173;
fifth day, 199
Supraoccipital plates, 236.
Supraoccipitals, 232, 233.
Surface view of embryo, 243, 244,
253> 256, 259, 262.
Sutures, 238.
T.
Tail, 125, 143.
Tail-fold, 35, 75, 125, 143.
Tarsal bones, 176.
Tarso-metatarsus, 176.
Testes, 167.
THOMSON, Allen, 6, 12.
Thyroid body, 133, 134.
Tibiale, 176.
TONGE, 190, 191, 215.
Tongue, 205.
Trabeculae cranii, 178, 179,227, 228.
Trachea, 131. .
Tread, 2.
Tubuli seminiferi, 167.
Tubuli uriniferi, 164:
'Tween brain, 91.
U.
Ulnare, 175.
Umbilical arteries, i53; duct, 143,
174; veins, 172, 210.
Ureter, 163.
Utriculus, 115.
INDEX.
V.
Vas deferens, 168.
Vascular area, 28, 56, 57, 84, 85; v.
layer, 45.
Veins; allantoic 210; cardinal 124;
of the liver, 171; hepatic 210;
jugular 207; mesenteric 171, 210;
omphalo-mesaraic 63, 69, 80, 170,
202; portal 210; umbilical 172,
210; vena cava inferior, 171, 207,
209; vena terminalis, 69; venae
advehentes, 171, 209, 210; veme
cavas, 209; venae revehentes, 171,
209, 210.
Venous circulation, 170, 21 1 ; venous
system, 169 172.
Ventricles, 79, 193; fourth ventricle
94-
Ventricular septum, 172, 173.
Vertebral arteries, 217; column, 152
155; plate, 54, 56.
Vesicle, germinal, 20 ; olfactory 1 1 7 ;
vesicle of the cerebral hemispheres,
78; optic vesicle 76, 77, 94, 95,
96 ; otic vesicle in; vesicle of the
third ventricle, 91.
Vestibule, in.
Visceral arches, 179.; clefts, 119, 204;
folds, 119.
Vis essentialis, 5.
Vitellin, 15.
Vitelliiie membrane, 13, 20.
Vitreous humour, 101.
Vomer, 237.
VON BAER, 5, 6, 7, 41, 64, 65, 71,
72, 78, 86, 91, 148, 150, 166, 173,
190, 194, 201, 202, 206, 213, 214,
215, 217, 219.
W.
WALDEYEK, 82, 149, 160, 161, 162,
163, 165, 167.
White of egg, 13.
White columns, 184, 185, 186, 189;
white matter, 184, 185, 186, 187,
189 ;* white yolk, 15.
Wing, 175.
WOLFF, 4, 5, 6, 71.
Wo'.ffiaii bodies, 139, 161 163, 167;
Wolffian duct, 73, 82, 83, 138, 139,
160 162, 168; Wolffian ridge, 143.
Y.
Yolk, 14 1 6. 202; yellow yolk, 14;
white yolk, 15.
Yolk-sac, 35.
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